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XMM-Newton (X-ray Multi-Mirror Mission-Newton) Observatory

Spacecraft   Launch   Sensor Complement   Ground Segment   Mission Status   References

XMM-Newton has been one of the most successful astronomy missions launched by ESA (European Space Agency). It exploits innovative use of replication technology for the X-ray reflecting telescopes that has resulted in an unprecedented combination of effective area and resolution. Three telescopes are equipped with imaging cameras and spectrometers that operate simultaneously, together with a co-aligned optical telescope. The key features of the payload are described, and the in-orbit performance and scientific achievements are summarized. 1) 2) 3) 4)

Background: An X-ray astronomy mission for the European Space Agency Horizon 2000 program was studied from the early 1980's, culminating in a mission presentation at an ESA workshop held in Lyngby, Denmark in June 1985. In the papers presented at this conference, the mission design contained 12 low-energy and 7 high-energy telescopes with a collecting area of 13000 cm2 and 10000 cm2 at 2 and 6 keV respectively. The scientific goal was to maximize the collecting area for spectroscopy, complementing the imaging science of the NASA AXAF program. When the report of the telescope working group was delivered in 1987, the consideration of practical constraints had reduced the number of telescopes to a more modest total of 7.

The mission was approved into implementation phase in 1994, and an improved observing efficiency achieved with a highly eccentric orbit allowed the number of telescopes to be reduced. The development of suitable mirrors involved parallel studies of solid nickel, nickel sandwich and carbon fiber technologies up to (as late as) early-1995. Soon afterwards the nickel electroforming replication technology was adopted, following the delivery of two successful mirror module demonstration models. The XMM flight model mirror modules were delivered in December 1998.

Mission objectives: Since Earth's atmosphere blocks out all X-rays, only a telescope in space can detect and study celestial X-ray sources. The XMM-Newton mission is helping scientists solve a number of cosmic mysteries, ranging from the enigmatic black holes to the origins of the Universe itself. Observing time on XMM-Newton is made available to the scientific community who apply to the regular announcements of opportunity on a competitive basis.

Mission name: The mission was initially known as XMM after its X-ray Multi-Mirror design, and was formally called the High Throughput X-ray Spectroscopy Mission because of its great capacity to detect X-rays. The name was later modified to XMM-Newton in honor of Sir Isaac Newton. — Announcing the new name on 9 February 2000, ESA's former Director of Science Prof. Roger Bonnet explained: "We have chosen this name because Sir Isaac Newton was the man who invented spectroscopy and XMM is a spectroscopy mission. The name of Newton is associated with the falling apple, which is the symbol of gravity and with XMM I hope that we will find a large number of black hole candidates which are of course associated with the theory of gravity. There was no better choice than XMM-Newton for the name of this mission".

A key aspect of the design was the simultaneous operation of 6 co-aligned instruments, the three EPIC (European Photon Imaging Camera) imaging X-ray cameras, the two RGS (Reflection Grating Spectrometer) grating X-ray spectrometers and the OM (Optical Monitor).

The XMM-Newton observatory provides unrivalled capabilities for detecting low surface brightness emission features from extended and diffuse galactic and extragalactic sources, by virtue of the large field of view of the X-ray telescopes and the high throughput yielded by the heavily nested telescope mirrors. In order to exploit the excellent EPIC data from extended objects, the EPIC background, now known to be higher than estimated pre-launch, needs to be understood thoroughly.

NASA cooperation: Besides having funded elements of the XMM-Newton instrument package, NASA also provides the NASA Guest Observer Facility (GOF) at NASA/GSFC (Goddard Space Flight Center). The GOF provides a clearing house for project-generated technical information and analysis software as well as budget support for U.S. astronomers who apply for XMM-Newton observation time. 5)

Some Science Highlights as of 2015

• Determined that Milky Way's Black Hole is believed to have woken up violently about 400 year ago and then turned off again about 100 years later.

• Identified the potential signatures of solar axioms, dark matter particle candidates.

• Measured the spin rate of a supermassive black hole for the first time in collaboration with NuSTAR.

• Acquired the first large-scale map of the dark matter and baryon distributions in the universe.

• Detected for the first time a switching X-ray emission when monitoring a highly variable pulsar - reopened the debate about the physical mechanisms powering the emission from pulsars.

• Discovered that the Orion Nebula contains a huge cloud of extremely hot gas, or plasma, heated to millions of degrees.

• Constructed the largest catalog of cosmic X-ray emitting objects.

• Showed that fierce winds from a supermassive black hole blow outward in all direction in collaboration with NuSTAR.

• Discovered 2XMM J083026+524133, the most massive cluster of galaxies seen in the distant Universe up to that time.

• Discovered the first definite detection of charge exchanged induced by X-ray emission on Mars.

• Acquired images of gamma-ray burst GRB 031203 that revealed the first detection of a time-dependent dust-scattered X-ray halo around a gamma-ray burst.

• Analyzed spectra of a distant active galaxy, 1H0707-495, which revealed two bright features of iron emission (the iron L and K lines) in the reflected X-rays that had never been seen together in an active galaxy.

Table 1: In orbit for more than 15 years, XMM-Newton has provided many insights into the workings of the universe, near and far. Here are a few examples (Ref. 5).

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Figure 1: Artist's rendition of the XMM-Newton spacecraft in orbit (image credit: ESA)

Spacecraft:

The XMM-Newton Observatory is a cornerstone mission of the European Space Agency's Horizon 2000 program, and is the largest scientific satellite it has launched to date. XMM-Newton is a three-axis stabilized spacecraft with a pointing accuracy of one arcsec. The satellite, which had a launch mass of 3800 kg, is made up of: a service module bearing the three X-ray mirror modules, propulsion and electrical systems, a long telescope tube and the focal plane assembly carrying the science instruments. The total length of XMM-Newton is 10 m, and its solar arrays give the satellite a 16 m span. The satellite was built under contract to ESA by the prime contractor Astrium, formerly Dornier Satellitensysteme (Friedrichshafen, Germany - part of DaimlerChrysler Aerospace), led an industrial consortium involving 46 companies from 14 European countries and one in the United States. Media Lario, Como, Italy, developed the X-ray mirror modules. - Although the nominal mission was for two years, XMM-Newton has been designed and built to perform well beyond that period. Mission operations have been extended until end-2018, with a mid-term review in 2016. 6) 7) 8)

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Figure 2: An exploded view of XMM, highlighting the spacecraft's modular configuration (image credit: ESA)

The XMM spacecraft has a conventional structure and thermal design. Due to the long focal length of the telescopes (7.5 m), the mirrors are far removed from the instruments. On the ground and during the launch, the structure has to maintain the integrity of the whole spacecraft. The thermal control does not make use of onboard software. In orbit, the functions of the structure and the thermal control are mixed. Their global common requirement is to relate and align the set of mirrors at one end of the spacecraft with the set of instruments at the other.

TT (Telescope Tube): The TT maintains the relative position between the FPA (Focal Plane Array) and the MSP (Mirror Support Platform). Due to its length of 6.80 m, the Telescope Tube is physically composed of two halves: the upper and lower tubes. The upper tube includes two reversible VOD (Venting and Outgassing Doors), and supports the OGB (Outgassing Baffle).

TCS (Thermal Control Subsystem): The XMM satellite relies on a combination of passive and active means of thermal control. The passive thermal control is mainly achieved by using classical highly-insulating multi-layer blankets. Typically, blankets are made internally of 20 double-sided aluminized layers separated by Dacron nets. The external layer of all blankets is made of carbon-loaded Kapton, which gives the satellite its characteristic black appearance. This kind of Kapton has been chosen because of its electrical conductivity, which avoids electrostatic-discharge problems. In addition, the thermo-optical properties of the black finish will not change during the satellite's ten-year lifetime, helping again to maintain temperature stability. 9)

The insulation performance that has been achieved by the XMM blankets is exceptionally good, especially for the large, undisturbed blankets that insulate the telescope tube. Together with the black lining of its internal surface, they keep the temperature gradient across the tube diameter small and stable. In fact, the measured temperature difference across the tube was only 3°C. The telescope tube is not equipped with heaters and its temperature control is purely passive.

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Figure 3: Photo of the insulated telescope tube (image credit: ESA)

Focal-plane assembly compartment: The FPA compartment, located on top of the telescope tube and which contains the payload cameras, is controlled during operations in a totally passive way. This is made possible by the power dissipated by the instruments, which remains fairly constant during the mission observation periods. Consequently, the compartment's heat losses are trimmed such that the dissipated power (about 150 W) can keep the temperatures at the required level. Whenever an instrument chain is switched off, an appropriate ‘substitution' heater line is switched on in order to replace the missing dissipated power and keep the heat power balance constant. In nonoperative and emergency modes, mechanical thermostats will switch these heaters if the temperature falls close to the non-operation temperature limits of the equipment.

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Figure 4: Photo of the FPA compartment which houses the payload cameras (image credit: ESA)

SVM (Service Module): The SVM, accommodated at the other end of the telescope tube, is also fully blanketed with the exception of panel radiators. On the Sun-side they are covered by mirror solar reflectors, while those on the anti-sun side of the satellite are painted white. Where passive measures are not sufficient to meet the temperature requirements, heaters controlled by thermostats are implemented. No on-board software is used to activate and control heaters. Ground control can configure the heater lines to be powered, while mechanical thermostats perform the actual heater switching. In a typical mission observation phase, about 330 W are dissipated by the equipment and 80 W are provided by the heaters to maintain an internal average temperature of 15°C.

The configuration of the spacecraft is shown in an exploded view in Figure 5. It comprises 4 main sections (Ref. 1):

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Figure 5: View of the XMM-Newton spacecraft subsystems, with external shrouds and structure removed for clarity (image credit: ESA)

FPA (Focal Plane Assembly): The FPA, consisting of a platform carrying the focal-plane instruments: two RGS readout cameras, an EPIC PN and two EPIC MOS imaging detectors, and electronics boxes. The EPIC and RGS instruments are fitted with radiators, to passively cool the CCDs (Charge Coupled Devices).

MPS (Mirror Support Platform): The MSP, carrying the three mirror assemblies, the OM and the two star-trackers.

SVM (Service Module): The SVM,carrying the spacecraft subsystems, the two solar-array wings, the TSS (Telescope Sun Shield) and the two S-band antennae. The functions provided by the Service Module are:

- the primary and secondary structures to interface with the launcher adapter, to support the subsystem units, and to interface with and support the MSP (Mirror Support Platform) and the Telescope Tube

- the thermal control to maintain the SVM units and equipment within specified temperature limits, and to provide a very strictly controlled thermal environment for the mirror assemblies via the MZCU (Mirror Thermal Control Unit)

- the AOCS (Attitude and Orbit Control System) for precise pointing/slewing in all operational modes, and for the performance of orbit acquisition/maintenance via the RCS (Reaction Control System) propulsion system

- the OBDH (On-Board Data Handling) for the decoding of ground telecommands, the distribution of ground or on-board commands, the sampling and formatting of telemetry data, and central on-board time distribution

- the RFS (Radio Frequency System), operating in S-band, ensuring communications (uplinking of telecommands, downlinking of telemetry) with the ground stations and providing a ranging mode for orbit determination

- the EPS (Electrical Power Subsystem) for the generation and distribution of regulated power to all equipment via a 28 V main bus.

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Figure 6: Exploded view of the XMM Service Module, with its side panels open. Thanks to its particular shape, with a large central hole of 2.10 m diameter, the XMM bus can be used to accommodate a large variety of payloads (image credit: ESA, Ref. 7)

For reasons largely related to the mission design lifetime XMM-Newton has no onboard data storage capacity, so all data are immediately downloaded to the ground in real time, as facilitated by the ground station visibility. Contact for continuous real-time interaction with the spacecraft over almost the entire orbit is provided by the ESA ground stations at Perth and Kourou (with Villafranca and Santiago as back-up).

Mission operations are managed through the ESA/ESOC (European Spacecraft Operations Center) in Darmstadt, Germany. The observations are managed and archived at the ESAC (European Space Astronomy Center), formerly known as VILSPA) at Villafranca, Spain. The science data are processed at the XMM-Newton Survey Science Center at the University of Leicester, England.

Telescope Tube: The XMM observatory has, at its heart, three large X-ray telescopes, which will provide a large collecting area (1430 cm2 each at 1.5 keV, and 610 cm2 each at 8.0 keV) with a spatial resolution of around 14-15 arcsec. 10) 11)

The long carbon fiber Telescope Tube (not shown in Figure 5), maintaining the FPA and the MSP separation. The upper half includes reversible venting and out-gassing doors and baffles. The three telescopes each consist of the following elements, as shown in Figure 7:

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Figure 7: XMM-Newton telescope assembly (image credit: ESA)

- Mirror Assembly Door, which closed and protected the X-ray optics and the telescope interior against contamination until operations began.

- Entrance Baffle, which provides the visible stray light suppression.

- XRB (X-ray Baffle) which blocks X-rays from outside the nominal field of view, which would otherwise reflect once on the hyperboloid section of the mirrors and would therefore cause stray light.

- MM (Mirror Module) the X-ray optics themselves (Figure 8). The XMM mirrors were developed and manufactured by ESA and Media Lario (Bosisio Parini, Italy) with the support of: APCO (Vevey, Switzerland) for the manufacture of the structural parts and the containers; BCV-Progretti (Milan, Italy) for the optical and structural analysis; Kayser Threde (Munich, Germany) for the mechanical design and the analysis. The optical and X-ray stray-light work was conducted by ESA and Dornier (Friedrichshafen, Germany).

- Electron Deflector (producing a toroidal magnetic field for diverting "soft" electrons), right behind the mirrors in the shadow of the MM spider.

- RGA (Reflection Grating Assembly), with a mass of 60 kg, only present on the backside of two out of three MMs. It deflects roughly half of the X-ray light to a strip of CCD detectors (RGS), offset from the focal plane.

- Exit Baffle, providing a thermal environment for the gratings.

A Mirror Module is a Wolter 1 type grazing incidence telescope with a focal length of 7.5 m and with a resolution of ~15 arcsec (on-axis Half Energy Width). Each consists of 58 nested mirror shells bonded at one end on a spider. Design details are summarized in Table 1.

Focal length

7500 mm

Resolution (0.1-12 keV) HEW (Half Energy Width)

15 arcsec (HEW), < 8 arcsec (FWHM)

Effective area (1 keV)

1500 cm2

Mirror diameter and thickness

Outermost:700 mm / 1.07 mm, Innermost: 306 mm / 0.47 mm

Mirror length

600 mm

Packing distance

1-5 mm

Number of mirrors

58

Reflective surface

Gold (250 nm layer)

Mirror Module mass

420 kg

Table 2: Summary design parameters of an XMM-Newton Mirror Module

The X-ray mirrors are thin monolithic gold-coated nickel shells. The mirror shell manufacturing is based on a replication process, which transfers a gold layer deposited on the highly polished master mandrel to the electrolytic nickel shell, which is electroformed on the gold layer. The mandrels are made out of initially double conical aluminum blocks coated with Kanigen nickel and then lapped to the exact shape and finally super-polished to a surface roughness better than 4 Å (0.4 nm). The process is represented graphically in Figure 9.

In order to allow rapid testing of the individual mirror shells and integrated mirror modules a special vertical test facility (using a UV beam) was developed at the Centre Spatial de Liège in Belgium. The X-ray testing of the integrated XMM-Newton mirror modules was performed at the Panter facility of the MPE (Max-Planck Institut für Extraterrestrische Physik) Garching, Germany.

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Figure 8: Optical design of the XMM Mirror Module with the EPIC and RGS detectors (image credit: ESA)

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Figure 9: Mirror production process (image credit: ESA)

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Figure 10: Photo of the XMM telescope during wide-angle stray-light testing at Dornier (Ottobrunn, Germany), image credit: ESA

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Figure 11: Photo of the Mirror Module entrance plane with the 58 X-ray mirrors (image credit: ESA)

 

EPS (Electrical Power Subsystem): The principal mission and spacecraft characteristics influencing the design of XMM's EPS were: 12)

- Power requirements: 1600 W in the sun, and 600 W in the eclipse phase of the orbit.

- Spacecraft geometry: Two separate PDUs (Power Distribution Units) to reduce harness mass, and to simplify system testing and AIV (Assembly, Integration and Verification), the Service and Focal-Plane Assembly Modules have dedicated the PDUs.

- Mirror stability: Leading to a dedicated MTCU (Mirror Thermal Control Unit).

The resulting EPS design is comprised of the following elements:

- A fixed two-wing deployable solar array for power generation.

- Two nickel-cadmium batteries.

- Two independent PDUs: one for the Focal-Plane Assembly (FPA PDU) and the other for the Service Module (SVM-PDU).

- A MTCU, dedicated to the thermal control of the mirror platform, Mirror Modules and Reflection Grating Assemblies.

- A PRU (Pyrotechnic Release Unit) for automatic activation of the release mechanisms for the solar arrays, and for the initial transmitter and AOCS activation.

The solar array has two wings, each with three rigid 1.94 m x 1.81 m panels, giving a total area of 21 m2 and a mass of 81.4 kg. The two wings are body-fixed and have a Sun incidence angle variation around normal of up to 28º. At end-of-life (EOL = 10 years), in the worst case, including one failed section, the solar array is required to provide 1600 W at 30 V at the interface connectors.

Battery: XMM has two identical 24 Ah nickel-cadmium batteries, each with 32 cells. Each 573 x 188 x 222 mm3 battery weighs 42 kg. To allow for cell short-circuits, the nominal energy budget is calculated with 31, rather than the full 32 cells. To allow for high peak power demands, a battery voltage higher than the bus voltage has been chosen, and battery reconditioning will be performed before each eclipse season.

MRU (Main Regulator Unit): The MRU provides a 28 V regulated main bus voltage, with protection to ensure uninterrupted operation even in the event of a single-point failure. During sunlit periods, the MRU provides power via S3Rs (Sequential Switching Shunt Regulators), as well as managing battery charging. In eclipse mode, the MRU controls the discharging of the two batteries to ensure correct current sharing. To reduce the power demand on the batteries and ensure that all non-essential loads are switched off during eclipse, an eclipse signal ECL is generated by the MRU and sent to the PDUs and MTCU. - The MRU provides 2 x 2 power lines for the SVM-PDU and the FPA-PDU, and 1 x 2 switched lines for the MTCU. For ground testing, it provides interfaces with solar-array and battery simulators.

OBDH (On-Board Data-Handling): OBDH is implemented in three internally redundant physical units: the CDMU (Central Data Management Unit) and two RTUs (Remote Terminal Units). The CDMU and one RTU are located on the Service Module of the spacecraft. The second RTU is installed on the FPA (Focal Plane Assembly). In addition to the RTUs and the CDMU, the OBDH includes six DBUs (Data Bus Units), which provide the scientific instruments with a digital interface to the data-handling services. 13)

The data processing in the CDMU is performed by the CTU (Central Terminal Unit) based on a MIL-STD-1750 microprocessor with 256 kwords of RAM. The packet handling functions, i.e. telemetry frame generation and telecommand frame decoding, are implemented in standard ASICs developed under ESA contracts, VCAs (Virtual Channel Assemblers), VCMs (Virtual Channel Mutiplexers) and PFDs (Packet Telecommand Decoders).

The users and all of the OBDH units are interconnected by the ESA OBDH bus, comprising a redundant set of interrogation bus and mono-directional response bus. The OBDH bus can transfer, depending on the command rate, approximately 200 kbit/s of telemetry data, which is roughly three times the XMM downlink data rate of 69.4 kbit/s (source packet level).

Packet protocol: Packet Terminals are connected to the OBDH via dedicated DBUs (Data Bus Units). The interface to the DBU, and thus the OBDH bus, is realized by an ASIC RBI (Remote Bus Interface). In order to have a common interface to all Packet Terminals, this specific RBI was imposed on all instruments and the AOCS (Attitude and Orbit Control Subsystem).

The RBI provides the CDMU with DMA (Direct Memory Access) to the processor memory of the Packet Terminals. In addition, the RBI accommodates registers for communication between the Packet Terminal and the OBDH and a register holding a copy of the onboard time.

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Figure 12: Architecture of XMM's OBDH subsystem (image credit: ESA)

RF subsystem: The RF subsystem is composed of three main blocks which are two Low Gain Antennas (LAG1 (+Z) and LAG2 (-Z)), two transponders and a Radio Frequency Distribution Network (RFDN) with two switches (SW-A and SW-T) to connect the transponders either to the LGA1 or LGA2. An S-Band transponder comprises three main modules: a diplexer, a receiver and a transmitter. The receiver assures the reception of signal in the range of 2025 to 2120 MHz and the phase demodulation of the telecommand signal and the ranging tones. The transmitter performs the modulation of the telemetry video signal and the ranging tones, as well as the power amplification of the output signal. The diplexer allows operating simultaneously the receiver and the transmitter with just a single RF connection (Ref. 70).

After the recovery from an antenna switch problem in 2008 it was agreed not to operate the faulty antenna switch (SW-A) of the RFDN anymore. A new strategy was implemented by switching between receivers and the corresponding transmitters. Apart from the SW-A the performance of the RF subsystem is very good. No degradation of any sub assembly was seen so far. The transmitter Latching Current Limiters (LCLs) that switch the transmitters on and off have been qualified for 50000 switchings. So far <900 switches for each transmitter have
been performed without any problems. The transmitter output level has not decreased significantly in comparison to the beginning of the mission.

RCS (Reaction Control Subsystem): XMM-Newton is equipped with a monopropellant RCS, with helium as pressurant and hydrazine as propellant. The propellant storage system consists of four tanks, three Auxiliary Tanks, which feed into the Main Tank, which in turn feeds the thrusters. The tanks of RCS are to load a total of 520 kg of hydrazine. The RCS consists of a 4 tank configuration, consisting of three Auxiliary Tanks with partial PMD (Propellant Management Device) feeding into one Main Tank with full PMD.

Autonomy: XMM is required to survive three days of ground-station outage. The autonomy on XMM is decentralized, meaning that all subsystems should manage their own survival and supply essential services to allow other subsystems to survive. Thus the central spacecraft autonomy function, often implemented in the data-handling subsystem, was not required for XMM. Recognizing this has allowed a substantial reduction in design and test expenditure by limiting the OBDH autonomy to protection against OBDH internal failures.

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Figure 13: The XMM PFM Lower Module in the Large Space Simulator at ESTEC (January 1999). The Telescope Sun Shield is deployed. The three Mirror Module doors and the Optical Monitor door are open (image credit: ESA) 14)

 

Launch: XMM was launched on December 10, 1999, via the first commercial Ariane-5 launch from Kourou, French-Guyana. It was the largest scientific European spacecraft to date; built and launched within the budget and schedule defined at approval. The overall mission cost was 689 MEuro (1999 economic conditions).

Orbit: HEO (Highly-elliptical Earth Orbit ). After launch, the spacecraft was placed into a 48 hour elliptical orbit around the Earth, with an inclination of 40º, a southern apogee at an altitude of 114 000 km, and a perigee altitude of 7000 km. The orbital parameters evolve as the mission progresses. As an example, the perigee altitude has varied between 6000 km and 22 000 km, while the apogee altitude has varied between 99 000 km and 115 000 km. However, the orbital period is always kept at 48 hours.

XMM-Newton's operational orbit was chosen for two reasons: First, the XMM-Newton instruments need to work outside the radiation belts surrounding the Earth. Second, a highly eccentric orbit offers the longest possible observation periods - less interrupted by the frequent passages in the Earth's shadow that occur in LEO (Low Earth Orbit). In addition, the orbital period of XMM-Newton is exactly two times the Earth rotation period to maintain optimal contact between XMM-Newton and the ground stations tracking the satellite. This allows XMM-Newton data to be received in real-time and for it to be fed to the Mission Control Centers.

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Figure 14: The operational XMM orbit (image credit: ESA)

 


 

Sensor complement: (EPIC, RGS, OM)

EPIC (European Photon Imaging Camera)

Three cameras, one at the primary focus of each mirror module, were produced by a consortium of ten institutes in four nations: the UK, Italy, France and Germany. The EPIC Principal Investigator (PI) is Dr. Steven Sembay of the University of Leicester, UK. He took over from Prof. Martin Turner of the X-ray Astronomy Group at Leicester University, who was the EPIC PI up to 6 May 2009. One of the cameras uses a new type of CCD (pn) developed by the Max Planck Institute of Extraterrestrial Physics in Garching and the Astronomical Institute in Tübingen, both in Germany.

EPIC comprises a set of three X-ray CCD cameras. Two of the cameras contain MOS CCD arrays (referred to as the MOS cameras). They are installed behind the X-ray telescopes that are equipped with the gratings of the RGS. The gratings divert about half of the telescope incident flux towards the RGS detectors such that about 44% of the original incoming flux reaches the MOS cameras. The EPIC instrument at the focus of the third X-ray telescope with an unobstructed beam; uses pn CCDs and is referred to as the pn camera (Figure 16).

The EPIC cameras perform extremely sensitive imaging observations over the telescopes FOV (Field of View) of 30 arcmin and in the energy range from 0.15 to 15 keV with moderate spectral (E/ΔE ~ 20-50). All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency. 15) 16)

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Figure 15: The CCDs of one of the MOS cameras in the cryostat (image credit: ESA)

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Figure 16: The CCDs of the pn camera: The picture shows the twelve chips mounted and the connections to the integrated preamplifiers (image credit: ESA)

EPIC MOS CCDs: The MOS CCDs, EEV type 22, have 600 x 600 pixels, each 40 µm x 40 µm; they are frame-transfer devices and front illuminated. One pixel covers 1.1 arcsec. The third phase electrode is open so that 40% of the area is only covered by 400 Å of silicon oxide. This improves the low energy response, giving appreciable sensitivity down to 150 eV. There being no buried channel makes this part of the pixel immune to channel damage such as that caused by soft protons interacting just below the surface of the pixel. This, combined with the rather thicker oxide and poly-silicon that covers the buried channel, contributes to the relative radiation hardness of the devices to soft protons.

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Figure 17: Representative images of the EPIC MOS camera acquired in the different operating modes. Left to Right: Full Frame Image, Large Window, Small Window, Timing (image credit: ESA)

The full frame integration time is 2.6 seconds, governed by the time taken to read out the 600 x 600 pixels with sufficiently low noise (<5 electrons). The CCDs are read out in sequence. The central CCD can be commanded independently of the others, which are commanded in pairs; the modes can be applied to the central CCD as described in Table 3 and Figure 17.

Central CCD mode

Imaging area pixels (arcmin)

Time resolution (s)

Full

600 x 600 (11 x 11)

2.6

Large window

300 x 300 (5.5 x 5.5)

0.6

Small window

100 x 100 (1.8*1.8)

0.3

Timing

None (1.8 arcmin projection)

~10-3

Table 3: Summary of EPIC MOS CCDs readout modes

The CCDs are cooled passively and maintained at their operating temperature using a three-stage radiator system combined with heaters. The minimum design temperature for the cooling system is –130 °C. There is provision for de-icing the radiators and CCDs by raising the temperature to near 0ºC and for annealing the CCDs by raising their temperature to +120ºC. Neither of these operations has so far needed to be performed. The nominal operating temperature of the CCDs at launch was –100 ºC, and this is maintained to within ± 0.5 º by the control electronics.

EPIC PN CCDs: The principle of sideward depletion in high resistivity silicon is the basis of a large variety of novel silicon detectors, such as silicon drift detectors, controlled drift detectors, active pixel sensors — and pn-CCDs. The essential part of the EPIC PN detector is the 10 cm silicon wafer divided in an array of 12 monolithically implanted pn-CCDs, developed and manufactured in a dedicated semiconductor laboratory . The array has a total effective size of 6 cm x 6 cm. The CCDs are arranged in four quadrants of three CCDs each. For redundancy reasons, each quadrant can be considered as a separate unit: it has its own power supplies, back contact, preamplifiers and event analyzer electronics and can be operated independently from the others. The single CCD has a dimension of 3 cm x 0.98 cm with pixels of 150 µm x 150 µm arranged in 200 rows and 64 channels. The pixel size corresponds to an angular resolution element of 4.1 arcsec. The 64 read out anodes of each CCD are connected via on chip JFETs (Junction Field Effect Transistor) to a 64 channel charge sensitive amplifier (CAMEX64B). The 12 x 64 output channels of the CCDs as well as the supply and bias voltages are connected via bond wires to a multilayer printed circuit board, which carries the charge sensitive amplifiers as well as filter circuits for supply and bias voltages. 17)

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Figure 18: Mechanical structure of the pn-CCD camera system. X-rays imaged through the telescope enter the detector from the bottom (image credit: EPIC Team)

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Figure 19: Readout modes of EPIC PN camera. Left to Right: Full window, Large window, Small Window, Timing/Burst (image credit: ESA)

CCD mode

Imaging area pixels (arcmin)

Time resolution (s)

Full

384 x 376 (26 x 27)

73

Large window

384 x 198 (26 x 13.6)

48

Small window

64 x 64 (4.4 x 4.4)

5.7

Timing

None (4.4 arcmin projection)

0.03

Table 4: Summary of EPIC PN CCDs readout modes. An additional special version of Timing mode provides even higher time resolution but with low duty cycle

Several read out modes can be selected for the pn-camera (Figure 19) to adjust the performance to the observation requirements (source flux). The full frame and window modes are the imaging modes. Timing and burst mode achieve better time resolutions at the expense of the coordinate in shift direction. In full frame mode, all 12 CCDs are read out sequentially within 70.25 ms, which corresponds to the time resolution in this mode. A CCD remains sensitive also during its read out cycle. This causes the so called 'out-of-time' events. These are events arriving during read out phase, when the charge content of the CCD is shifted to the read out anodes. The out-of-time events are distributed along the shift direction and get therefore a wrong coordinate in this direction. In full frame mode their fraction of 6.6% is given by the ratio between the read out time of one CCD and the read out time of all CCDs including 'warm up' times for preamplifiers.

EPIC Filter Wheel: Because CCDs are light sensitive, two kinds of light filter are provided. Their design is a compromise between the need to prevent optical and UV photons from reaching the CCD plane, and the need to absorb as few X-ray photons as possible, especially at the lowest X-ray energy. The thin and medium filters comprise an unsupported polyimide film, 160 nm thick, on with a single layer of aluminum; 40 nm on the thin filter and 800 nm on the medium. The transmission of visible light is approximately 10-2 and 10-4, respectively. The thick filter is constructed on an unsupported polypropylene film 330 nm thick, with 160 nm of aluminum and 40 nm of tin to block the UV that would otherwise be transmitted by the polypropylene. Because of the large diameter (~65 mm) and delicacy of these filters, it was felt unwise to launch them other than under vacuum, thus minimizing the acoustic load, transferred to the filter from the rocket. Late in the campaign, a leak developed in one of the cameras that prevented the vacuum from being maintained in the long period between last access for pumping and the launch. This camera was filled with helium at atmospheric pressure with a blow-off valve to vent the camera during ascent. This system worked effectively to reduce acoustic load (cf. possible load with air) and the filters suffered no damage during launch. During radiation belt passage, and at all other times deemed unsafe for operation, the filter wheels are kept in the closed position.. There are ‘Closed-Cal' observations used to monitor the long-term degradation of the CCD performance, by closing the filter wheel with a calibration source illuminating the CCDs. Once the radiation level is deemed to be safe, the filter wheel is moved to the appropriate filter position and the observation begins.

 

RGS (Reflection Grating Spectrometer)

Two of XMM-Newton's three mirror modules are equipped with an RGS grating array that disperses about 40 per cent of the light to an RGS detector in a secondary focus. The RGS Principal Investigator is Dr. Jelle Kaastra of the High-Energy Astronomy division SRON (Space Research Organization Netherlands), Utrecht, The Netherlands. He took over from Prof. Bert Brinkman, SRON, Utrecht, The Netherlands, who was the first RGS PI until 1 April 2002.

Optical design: The RGS design is illustrated in Figure 20. It incorporates an array of reflection gratings (RGA, aka grating stack) placed in the converging beam of an XMM-Newton telescope mirror module, this is done for two out of the three mirror modules on XMM-Newton. The grating stack consists of 182 precisely aligned reflection gratings which in total intercept about half the light emanating from the telescope. The undeflected light passes through and is collected by the EPIC instrument in the telescope focal plane. The individual gratings are located on a toroidal Rowland surface, formed by rotating the Rowland circle about an axis passing through the telescope and spectroscopic foci, as illustrated in the left panel of Figure 20. The gratings are slightly trapezoidal, since their edges lie along rays converging on the telescope focus.

While the field of view in the cross dispersion direction is determined by the width of the CCDs, the spatial resolution in this direction is largely determined by the imaging properties of the mirror. The situation in the dispersion direction is more complex as the source extent affects the wavelength resolution.

Nine large format back-illuminated CCDs are located on the Rowland circle to detect the dispersed spectra in single photon counting mode. The position of the X-ray on the detector is related to the X-ray wavelength through the dispersion equation. First, second and higher order spectra are overlapping on the detectors, but are easily separated by using the inherent energy resolution of the CCDs.

RGA (Reflection Grating Array): A RGA contains 182 identical gratings. The gratings are mounted at grazing incidence in the in-plane, or classical configuration, in which the incident and diffracted X-rays lie in a plane that is perpendicular to the grating grooves. Because the beam is converging, the gratings are oriented so that the angle of the incident X-ray at the center of the grating, α, is the same for all gratings in the array. In addition, the gratings all lie on the Rowland circle, which also contains the telescope focus and the spectroscopic focus for the blaze wavelength. In this configuration, aberrations, which would otherwise be introduced by the arraying, are eliminated. The telescope aperture is filled by rotating the Rowland circle about an axis passing through the telescope and spectroscopic blaze foci. In all, each RGA contains six rows of gratings.

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Figure 20: Optical design of the RGS (not to scale). X-rays, indicated by red arrows, enter from the top. Numerical values for a few key dimensions and angles are indicated (linear dimensions in mm, angles in degrees), image credit: ESA

Each grating measures about 10 cm x 20 cm (Figure 21). These large gratings need to be very flat in the long (i.e. dispersion) direction, since any non-flatness translates directly into a degradation of the resolution. At the same time, the grating substrates need to be very thin in order to minimize the obstruction of the direct beam by the gratings. The grating substrates consist of 1mm SiC face sheets with five stiffening ribs at the back, running in the direction of the X-ray beam (Figure 21). The face sheets are fabricated to 1 λ (634.8 nm) and 10 λ flatness in the long and the short direction, respectively. The gratings are replicated from a mechanically ruled master and are covered with a 200 nm gold coating. The groove density varies slightly (± 10 %) over the length of the gratings, to correct for aberrations associated with the converging beam. The groove density is ~646 grooves/mm at the center.

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Figure 21: Schematic drawing of a grating, including some of the key dimensions and angles (α=1.5762, β=2.9739 (for blaze wavelength of 1.5 nm, γ=2.2751, δ=0.6989, angles all in degrees).

The grating array support structure is made out of vacuum hot-pressed beryllium, which was selected for its low specific mass and good stability over the operational temperature range (10-30 ºC). To obtain the desired resolution, it is essential that all gratings are properly aligned (with 1 µm tolerance on the position of any grating corner).

RGS Focal plane camera: The dispersed spectrum is integrated on nine large format back illuminated CCDs. These nine CCDs are mounted in a row, following the curvature of the Rowland circle (see top two panels of Figure 22). To reduce the dark current and improve general performance the CCD's are cooled to -80 ºC. An increase in CCD charge transfer inefficiency (CTI) due to radiation damage can be reduced by lowering the temperature to -120 ºC. Cooling is accomplished by a two-stage radiator, facing deep space. The CCD bench is housed internally to three nested thermal shells around the CCD bench where the first shield also contains four internal calibration sources. The nine CCD chips are back-illuminated EEV devices with an image and storage section of 384 x 1024 pixels each and a pixel size of 27 µm by 27 µm and the length of the CCD assembly (253 mm) covers the first order dispersion of the 0.6 – 3.8 nm wavelength range. The instrument can be operated in a spectroscopy, high time resolution and diagnostic mode.

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Figure 22: CCD bench (right most two panels) as well an enlarged view of two adjacent CCDs (left panel). The dead space indicated is present between each pair of CCD's. The figure is not to scale (image credit: ESA)

 

OM (Optical Monitor)

OM is an optical/ultraviolet telescope, co-aligned with the main X-ray telescope, gives the XMM-Newton mission a multi-wavelength capacity. The Mullard Space Science Laboratory (MSSL) UK has supplied this 30 cm aperture Ritchey-Chrétien telescope (with a 180-600 nm spectral range). The OM Principal Investigator is Prof. Keith Mason.

The Optical/UV Monitor Telescope (XMM-OM) is mounted on the mirror support platform alongside the X-ray mirror modules. It provides coverage between 170 nm and 650 nm of the central 17 arcmin square region of the X-ray field of view, permitting routine multi-wavelength observations of XMM targets simultaneously in the X-ray and ultraviolet/optical bands A schematic of the XMM-OM is shown in Figure 23. It consists of two modules which are physically separated on the spacecraft; the telescope module and dual redundant digital electronics modules. The Telescope Module contains the telescope optics and detectors, the detector processing electronics and power supply. The Digital Electronics Module houses the ICU (Instrument Control Unit), which handles communications with the spacecraft and commanding of the instrument, and the Data Processing Unit, which preprocesses the data from the instrument before it is telemetered to the ground.

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Figure 23: System block diagram of Optical Monitor (image credit; ESA)

The telescope is of a Ritchey-Chrétien design and has a clear aperture of 30 cm. The f/ratio of the primary mirror is f/2.0, which is modified by the secondary to f/12.7, i.e. a focal length of about 3.8 m. The light beam is intercepted by a 45º flat mirror, located behind the primary mirror, which can be rotated to direct the beam to one of two redundant filter-wheel/detector assemblies. The format of the detector is 2048 x 2048 pixels with each pixel 9.5 µm square. The field of view is 24 arcmin on the diagonal. In order to flatten the intrinsically curved focal plane, the detector window has been made concave (thinner at the center), and the filters are weakly figured.

The detector is a microchannel plate (MCP)-intensified CCD (MIC). Incoming photons are converted into photoelectrons in an S20 photocathode deposited on the inside of the detector window. The photoelectrons are proximity focused onto a stack of three microchannel plates, which amplifies the signal by a factor of a million, through a bias of ~1.8 kV. The resulting electrons are converted back into photons by a P46 phosphor screen. Light from the phosphor screen is passed through a fiber taper which reducing the image scale to compensate for the difference in physical size between the microchannel plate stack and the fast-scan CCD used to detect the photons.

In each detector there is a filter wheel. The filter wheel has 11 apertures, one of which is blanked off to serve as a shutter, preventing light from reaching the detector. Another seven filter locations house lenticular filters, six of which constitute a set of broad band filters for color discrimination in the UV (UVW1, UVM2, UVW2) and optical (U, B, V) between 180 nm and 580 nm. The seventh is a "white light" filter which transmits light over the full range of the detector to give maximum sensitivity to point sources (Figure 24). The remaining filter positions contain two grisms, one optimized for the UV and the other for the optical range, and a x4 field expander.

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Figure 24: OM Filter transmissions (image credit: ESA)

The signal from the CCD detector is processed by fast electronics near the detector head to extract information which is then transmitted to the DPU (Data Processing Unit) in the DEM (Digital Electronics Module). Each DEM contains an ICU (Instrumental Control Unit) and a DPU. The ICU commands the XMM-OM and handless communications between the XMM-OM and the spacecraft. The DPU is an image processing computer that digests the raw data from the instrument and applies a non-destructive compression algorithm ("tiered block word length" type) before the data are telemetered to the ground via the ICU. The DPU supports two main science data collection modes, which can be used simultaneously. The DPU autonomously selects up to 10 guide stars from the full OM image and monitors their position in detector coordinates at intervals that are typically set in the range 10-20 seconds, referred to as a tracking frame. These data provide a record of the drift of the spacecraft during the observation accurate to ~ 0.1 arcsec.

The OM telescope module consists of a stray light baffle and a primary and secondary mirror assembly. The separation of the primary and secondary mirrors is critical to achieving the image quality of the telescope. The separation is maintained to a level of 2 µm by Invar support rods that connect the secondary spider to the primary mirror mount, and by maintaining an isothermal condition through distributing the detector electronics heat with heat pipes.

 


 

XMM Ground Segment:

The ground segment consists of all of the infrastructure and systems needed on Earth to communicate with, monitor and control the XMM satellite in real time, as well as to gather, process and archive the scientific data harvested by the X-ray cameras on board.

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Figure 25: The overall mission architecture (image credit: ESA, Ref. 7)

The major components of the XMM ground segment are:

1) The ground stations to track the satellite and communicate at S-band with the on-board transponders. During the operational scientific part of the mission, two ESA ground stations are used: Perth in Australia and Kourou (the ‘Diane' station) in French Guiana. These stations were selected as offering near-complete coverage of the XMM orbit, with its apogee in the Southern-Hemisphere. During XMM's first 10 days in space – the so-called LEOP (Launch and Early Orbit Phase) – a third ESA station will be used in addition, namely Villafranca in Spain.

2) The MOC (Mission Operations Center) located at ESOC in Darmstadt (Germany). The MOC is responsible for monitoring and controlling the satellite: all telecommands will be sent from the MOC, and all telemetry will be received at the MOC.

The main elements of the Mission Operations Center are:

• The XMCS (XMM Mission Control System), in charge of receiving, decoding and processing the telemetry, as well as assembling the telecommands to the satellite.

• The XMM Simulator (called the MOC-SIM), a software model of the satellite platform, used for pre-validation of commands and procedures.

• The FDS (Flight Dynamics System), in charge of orbit determination and attitude reconstitution. The FDS also defines all of the attitude and orbit control maneuvers in an optimal way to maximize the duration of the scientific observations, whilst still respecting the XMM-imposed in-orbit constraints, such as avoidance of bright celestial bodies in the telescope field of view.

• The SOC (Science Operations Center) at ESAC ((European Space Astronomy Center), is located at Villafranca del Castillo near Madrid, Spain. The SOC is responsible for preparing all scientific observations, and analyzing and processing the corresponding scientific data. The main elements of the SOC are:

- The XMM Science Control System (XSCS) in charge of defining the planning of the scientific observations and monitoring their execution

- The XMM Simulator (called the SOC-SIM), a software model of the satellite experiments, used for validation of ground procedures, as well as validation of instrument on-board software

• The XMM Archive Management System (AMS), which stores and allows retrieval of all XMM data and associated products.

• The XMM SSC ( Science Survey Center) at Leicester University, United Kingdom, also plays a role. It performs pipeline processing of all XMM science data to identify and categorize all X-ray sources detected by XMM instruments.

- The XMM-Newton SSC Survey Science Center consortium was selected by ESA in early 1996. The SSC is an international collaboration involving a consortium of 8 institutions in the UK, France and Germany, together with 7 Associate Scientists. The SSC's role in facilitating the exploitation of the XMM-Newton serendipitous survey. 18)

Normal operations allow coverage of the observational periods of spacecraft orbit with only two ground stations (Perth and Kourou), however there is a seasonally varying gap close to spacecraft apogee of order 1 hour, during which no science operations can be conducted. This implies that uninterrupted exposures of about 65 ks can be implemented at the moment. It is anticipated that by the end of the year 2000, full orbit coverage can be achieved by using an additional operational antenna (Ref. 2).

Packets of science data from the 6 instruments, are multiplexed with their housekeeping data and spacecraft telemetry, and transmitted to ground at a rate of 64 kbit/s maximum. Spacecraft data are merged with earth reception time references, and transmitted via ground telecommunications links to the MOC (Mission Operations Center) at Darmstadt. The MOC monitors real-time payload health, performs commanding of operations, and provides additional analysis of spacecraft data to provide (for example) the Attitude History Files for each science observation. The science data are further transmitted to the SOC (Science Operations Center) at Villafranca, where science Quick Look Analysis is performed. The SOC then processes the data into the ODFs (Observation Data Files) for transmission to the science community.

The XMM-Newton project is using the ground stations at Perth (Australia), Kourou (French Guiana), Villafranca (Spain) and Santiago (Chile), according to Ref. 70).

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Figure 26: The data flow in the XMM-Newton ground segment (image credit: ESA)

 

Minimize Mission Status

Many of the following entries were found on the XMM-Newton 'Welcome to the XMM-Newton HelpDesk,' page of ESA under News: 19)

• October 30, 2017: ESA and NASA space telescopes have revealed that, unlike Earth's polar lights, the intense auroras seen at Jupiter's poles unexpectedly behave independently of one another. 20)

- Auroras have been seen in many places, from planets and moons to stars, brown dwarfs and a variety of other cosmic bodies. These beautiful displays are caused by streams of electrically charged atomic particles – electrons and ions – colliding with the atmospheric layers surrounding a planet, moon or star. Earth's polar lights tend to mirror one another: when they brighten at the North pole, they generally brighten at the South pole, too.

- The same was expected of auroras elsewhere, but a new study, published today in Nature Astronomy, reveals that those at the gas giant Jupiter are much less coordinated.

- The study used ESA's XMM-Newton and NASA's Chandra X-ray space observatories to observe the high-energy X-rays produced by the auroras at Jupiter's poles. While the southern auroras were found to pulse consistently every 11 minutes, those at the planet's north pole flared chaotically. 21)

- "These auroras don't seem to act in unison like those that we're often familiar with here on Earth," says lead author William Dunn of University College London's MSSL (Mullard Space Science Laboratory), UK, and Harvard-Smithsonian Center for Astrophysics, USA.

- "We thought the activity would be coordinated through Jupiter's magnetic field, but the behavior we found is really puzzling. It's stranger still considering that Saturn – another gas giant planet – doesn't produce any X-ray auroras that we can detect, so this throws up a couple of questions that we're currently unsure how to answer. Firstly, how does Jupiter produce bright and energetic X-ray auroras at all when its neighbor doesn't, and secondly, how does it do so independently at each pole?"

- With the data at hand, William Dunn and colleagues identified and mapped X-ray hot spots at Jupiter's poles. Each hot spot covers an area half the size of Earth's surface.

- As well as raising questions about how auroras are produced throughout the cosmos, Jupiter's independently pulsing auroras suggest that there is far more to understand about how the planet itself produces some of its most energetic emissions.

- Jupiter's magnetic influence is colossal; the region of space over which the Jovian magnetic field dominates – the magnetosphere – is some 40 times larger than Earth's, and filled with high-energy plasma. In the outer edges of this region, charged particles ultimately from volcanic eruptions on Jupiter's moon, Io, interact with the magnetic boundary between the magnetosphere and interplanetary space. These interactions create intense phenomena, including auroras.

- "Charged particles have to hit Jupiter's atmosphere at exceptionally fast speeds in order to generate the X-ray pulses that we've seen. We don't yet understand what processes cause this, but these observations tell us that they act independently in the northern and southern hemispheres," adds Licia Ray, from Lancaster University, UK, and a co-author.

- The asymmetry in Jupiter's northern and southern lights also suggests that many cosmic bodies that are known to experience auroras – exoplanets, neutron stars, brown dwarfs and other rapidly-rotating bodies – might produce a very different aurora at each pole.

- Further studies of Jupiter's auroras will help to form a clearer picture of the phenomena produced at Jupiter; auroral observing campaigns are planned for the next two years, with X-ray monitoring by XMM-Newton and Chandra and simultaneous observations from NASA's Juno, a spacecraft that started orbiting Jupiter in mid-2016.

- ESA's JUICE (JUpiter ICy moons Explorer) will arrive at the planet by 2029, to investigate Jupiter's atmosphere and magnetosphere. It, too, will observe the auroras and in particular the effect on them of the Galilean moons.

- "This is a breakthrough finding, and it couldn't have been done without ESA's XMM-Newton," adds Norbert Schartel, ESA project scientist for XMM-Newton. "The space observatory was critical to this study, providing detailed data at a high spectral resolution such that the team could explore the vibrant colors of the auroras and figure out details about the particles involved: if they're moving fast, whether they're an oxygen or sulphur ion, and so on. Coordinated observations like these, with telescopes such as XMM-Newton, Chandra and Juno working together, are key in exploring and further understanding environments and phenomena across the Universe, and the processes that produce them."

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Figure 27: The Hubble Space Telescope depicts Jupiter (image credit: NASA, ESA, and J. Nichols (University of Leicester))

Legend to Figure 27: Jupiter experiences intense auroras that rage constantly, created as charged particles from the Sun and the Jovian moon Io stream towards the gas giant and interact with its atmosphere. - A new study has revealed that the planet's northern and southern lights behave and pulse independently of one another–an unexpected finding given the behavior of Earth's auroras, which tend to mirror one another (when they brighten at the North pole, they generally brighten at the South pole too). - This image, from the NASA/ESA Hubble Space Telescope, shows such an aurora on Jupiter. It is a composite of two separate sets of Hubble observations–one of the polar aurora itself, photographed in the ultraviolet in June 2016, and an optical view of the full disc of Jupiter, taken in April 2014.

• September 6, 2017: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to just how hospitable a star system could be. A team of researchers looked at 24 stars similar to the Sun, each at least one billion years old, and how their X-ray brightness changed over time. 22) 23) 24)

- High levels of magnetic activity can produce bright X-rays and ultraviolet light from stellar flares. Strong magnetic activity can also generate powerful eruptions of material from the star's surface. Such energetic radiation and eruptions can impact planets and could damage or destroy their atmospheres, as pointed out in previous studies, including Chandra work reported in 2011 and 2013.

- Since stellar X-rays mirror magnetic activity, X-ray observations can tell astronomers about the high-energy environment around the star. In the new study the X-ray data from Chandra and XMM-Newton revealed that stars like the Sun and their less massive cousins calm down surprisingly quickly after a turbulent youth.

- "This is good news for the future habitability of planets orbiting Sun-like stars, because the amount of harmful X-rays and ultraviolet radiation striking these worlds from stellar flares would be less than we used to think," said Rachel Booth, a graduate student at Queen's University in Belfast, UK, who led the study.

- To understand how quickly stellar magnetic activity level changes over time, astronomers need accurate ages for many different stars. This is a difficult task, but new precise age estimates have recently become available from studies of the way that a star pulsates using NASA's Kepler and ESA's CoRoT missions. These new age estimates were used for most of the 24 stars studied here.

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Figure 28: GJ 176: A Sun-like star more than a billion years old (image credit: NASA/CXC/Queens Univ. of Belfast/R. Booth, et al.; Illustration: NASA/CXC/M. Weiss)

Legend to Figure 28: This artist's illustration depicts one of these comparatively calm, older Sun-like stars with a planet in orbit around it. The large dark area is a "coronal hole", a phenomenon associated with low levels of magnetic activity. The inset box shows the Chandra data of one of the observed objects, a two billion year old star called GJ 176, located 30 light years from Earth.

• July 10, 2017: Highly obscured and rapidly growing SMBH (Super-Massive Black Holes), known as AGN (Active Galactic Nuclei), might represent the key phase when SMBH accreted most of their mass and when the relationship between galaxies and their central SMBHs was established. A new study by an international team of astronomers led by Silvia Mateos from the Instituto de Física de Cantabria (CSIC-UC) in Spain, now suggests that many of the brightest SMBH may be escaping our detection as they hide in heavily obscured environments. 25)

- A large observational effort has been devoted to reveal the obscured AGN phase across the history of the Universe. Oddly enough, most AGN searches, mainly at X-ray wavelengths, report that obscured AGN (optically classified as type 2) become rarer with increasing accretion power. The receding torus model hypothesis is often invoked to explain this lack of luminous highly obscured AGN. It postulates that the most luminous AGN erode the obscuring material located in the vicinity of the SMBH, the so-called "dusty torus", reducing its covering factor. As a result, higher luminosity AGN should be rarely hidden from our view by the dusty torus.

- A completely different picture emerges now. Based on observations from both ground and space instruments, including spectroscopic observations with ISIS and ACAM on the William Herschel Telescope (WHT) and the European Photon Imaging Camera onboard the XMM-Newton observatory, the team led by Mateos built a sample of approximately 200 X-ray selected AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). Using models that self-consistently reproduce the emission from dust in the torus heated by the AGN, they determined for each AGN in their sample the fraction of the sky around the AGN central engine that is obscured, i.e., the geometrical covering factor of the dusty torus. The combined torus covering factor over the entire AGN population is precisely the intrinsic fraction of type 2 AGN.

- By equating the distribution of torus covering factors to the observed fraction of type 2 AGN in BUXS the team finds that many type 2 AGN with high covering factor tori have escaped detection in X-rays. Most importantly, these elusive AGN are increasingly numerous at higher AGN luminosities. When they account for the "missing" AGN, the team finds an intrinsic fraction of type 2 AGN of 58% with no (or at most a weak) dependence with AGN luminosity over three orders of magnitude in luminosity.

- According to Silvia Mateos, who led the research: "Clearly, the impact of the enormous AGN accretion power on the geometry of the nuclear material surrounding the SMBH is not as strong as we thought. Instead, a large fraction of luminous rapidly growing SMBH are so deeply embedded that they are escaping X-ray detection". 26)

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Figure 29: Type-2 AGN fraction vs. torus covering factor f2 for objects with 1042 <LX <1043 (top), 1043 <LX < 1044 (middle), and 1044 LX < 1045 erg s-1 (bottom). Filled circles are the observed type-2 AGN fractions in BUXS. Open squares are the best-fit models to the 1:1 relations (black solid lines) obtained by allowing a population of non-detected type-2 sources. The insets show the assumed f2 distribution of these missed sources (image credit: International Study Team)

• June 6, 2017: This blue ‘ball of strings' actually records 2114 movements made by ESA's XMM-Newton space telescope as it shifted its gaze from one X-ray object to another between August 2001 and December 2014 (Figure 30). — Orbiting in space since 1999, XMM-Newton is studying high-energy phenomena in the Universe, such as black holes, neutron stars, pulsars and stellar winds. 27)

- Even when moving its focus between objects, the space telescope collects scientific data, revealing X-ray sources across the entire sky. After correcting for overlaps between slews, 84% of the sky has now been covered.

- The plot is in galactic coordinates such that the center of the plot corresponds to the center of the Milky Way. The slew paths pass predominantly through the ecliptic poles, indicated by the density of overlapping slew paths to the top left and bottom right.

- Over 5000 papers have been published on XMM-Newton results to date. Scientists are also looking forward to the next generation of X-ray satellite, such as ESA's ATHENA (Advanced Telescope for High-ENergy Astrophysics), which is expected to be launched towards the end of the next decade.

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Figure 30: The image was created as part of the XMM-Newton Slew Survey Catalog release in March 2017, and which was featured as our Space Science Image of the Week last month (image credit: ESA/XMM-Newton/A. Read/R. Saxton , CC BY-SA 3.0 IGO)

• May 15, 2017: This colorful, seemingly abstract artwork is actually a map (Figure 31) depicting all the celestial objects that were detected in the XMM-Newton slew survey between August 2001 and December 2014. 28)

- Orbiting Earth since 1999, XMM-Newton is studying high-energy phenomena in the Universe, such as black holes, neutron stars, pulsars and stellar winds. But even when moving between specific targets, the space telescope collects scientific data.

- The plot is in galactic coordinates such that the center of the plot corresponds to the center of the Milky Way. High-energy sources along the center of the Milky Way include the famous black hole Cygnus X-1, and Vela X-1, a binary system comprising a neutron star consuming matter from a supergiant companion.

- Several star-and-black hole binary systems are also captured, including objects identified as GRS 1915+105, 4U 1630-47 and V 4641 Sgr.

- Two clusters of sources, one to the top left and one to the bottom right, correspond to the ecliptic poles. Objects above and below the plane of our Galaxy are predominantly external galaxies that are emitting X-rays from their massive black holes.

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Figure 31: The map shows the 30,000 sources captured during 2114 of these slews. Because of overlapping slew paths, some sources have been observed up to 15 times, and 4924 sources have been observed twice or more. After correcting for overlaps between slews, 84% of the sky has been covered. The plot is color-coded such that sources of a lower energy are red and those with a higher energy are blue. In addition, the brighter the source, the larger it appears on the map (image credit: ESA/XMM-Newton / R. Saxton / A.M. Read)

• May 10, 2017, Astronomers produced this dramatic new, highly-detailed image of the Crab Nebula (Figure 32) by combining data from telescopes spanning nearly the entire breadth of the electromagnetic spectrum, from the long waves seen by the Karl G. Jansky VLA (Very Large Array) to the extremely short waves seen by the orbiting Chandra X-Ray Observatory. 29) 30)

- The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, is some 6,500 light-years from Earth. At its center is a superdense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams of radio waves and light — a pulsar. The nebula's intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.

- The new VLA, Hubble, and Chandra observations all were made at nearly the same time in November of 2012. A team of scientists led by Gloria Dubner of IAFE, the National Council of Scientific Research (CONICET), and the University of Buenos Aires in Argentina then made a detailed analysis of the newly-revealed details in a quest to gain new insights into the complex physics of the object. They are reporting their findings in the Astrophysical Journal. 31)

- New details from the study show interactions between fast-moving particles and magnetic fields similar to structures seen on the Sun, other features seen to appear at multiple wavelengths, and structures that may indicate features near the star before it exploded. Two separate jets of material from near the pulsar appear in the X-ray and the radio images.

- "Comparing these new images, made at different wavelengths, is providing us with a wealth of new detail about the Crab Nebula. Though the Crab has been studied extensively for years, we still have much to learn about it," Dubner said.

- The NRAO (National Radio Astronomy Observatory) is a facility of the NSF (National Science Foundation), operated under cooperative agreement by Associated Universities, Inc.

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Figure 32: This image combines data from five different telescopes: The VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-Ray Observatory (X-ray) in purple (image credit: G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; NRAO/AUI/NSF; A. Loll et al.; T. Temim et al.; F. Seward et al.; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI)

• March 1, 2017: ESA and NASA space telescopes have made the most detailed observation of an ultra-fast wind flowing from the vicinity of a black hole at nearly a quarter of the speed of light. Outflowing gas is a common feature of the supermassive black holes that reside in the center of large galaxies. Millions to billions of times more massive than the Sun, these black holes feed off the surrounding gas that swirls around them. Space telescopes see this as bright emissions, including X-rays, from the innermost part of the disc around the black hole. 32)

- Occasionally, the black holes eat too much and burp out an ultra-fast wind. These winds are an important characteristic to study because they could have a strong influence on regulating the growth of the host galaxy by clearing the surrounding gas away and therefore suppressing the birth of stars.

- Using ESA's XMM-Newton and NASA's NuSTAR telescopes, scientists have now made the most detailed observation yet of such an outflow, coming from an active galaxy identified as IRAS 13224–3809. The winds recorded from the black hole reach 71 000 km/s – 0.24 times the speed of light – putting it in the top 5% of fastest known black hole winds.

- XMM-Newton focused on the black hole for 17 days straight, revealing the extremely variable nature of the winds. "We often only have one observation of a particular object, then several months or even years later we observe it again and see if there's been a change," says Michael Parker of the Institute of Astronomy at Cambridge, UK, lead author of the paper published in Nature this week that describes the new result. "Thanks to this long observation campaign, we observed changes in the winds on a timescale of less than an hour for the first time." 33)

- The changes were seen in the increasing temperature of the winds, a signature of their response to greater X-ray emission from the disc right next to the black hole.

- Furthermore, the observations also revealed changes to the chemical fingerprints of the outflowing gas: as the X-ray emission increased, it stripped electrons in the wind from their atoms, erasing the wind signatures seen in the data. "The chemical fingerprints of the wind changed with the strength of the X-rays in less than an hour, hundreds of times faster than ever seen before," says co-author Andrew Fabian, also from the Institute of Astronomy and principal investigator of the project. "It allows us to link the X-ray emission arising from the infalling material into the black hole, to the variability of the outflowing wind farther away."

- "Finding such variability, and finding evidence for this link, is a key step in understanding how black hole winds are launched and accelerated, which in turn is an essential part of understanding their ability to moderate star formation in the host galaxy," adds Norbert Schartel, ESA's XMM-Newton project scientist.

- The brightness of an active galactic nucleus is set by the gas falling onto it from the galaxy, and the gas infall rate is regulated by the brightness of the active galactic nucleus; this feedback loop is the process by which supermassive black holes in the centers of galaxies may moderate the growth of their hosts.

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Figure 33: Artist impression illustrating a supermassive black hole with X-ray emission emanating from its inner region (pink) and ultrafast winds streaming from the surrounding disk (purple), image credit: ESA

• February 21, 2017: ESA's XMM-Newton has found a pulsar – the spinning remains of a once-massive star – that is a thousand times brighter than previously thought possible (Figure 34). The pulsar is also the most distant of its kind ever detected, with its light travelling 50 million light-years before being detected by XMM-Newton. 34) 35)

- Pulsars are spinning, magnetized neutron stars that sweep regular pulses of radiation in two symmetrical beams across the cosmos. If suitably aligned with Earth these beams are like a lighthouse beacon appearing to flash on and off as it rotates. They were once massive stars that exploded as a powerful supernova at the end of their natural life, before becoming small and extraordinarily dense stellar corpses.

- This X-ray source is the most luminous of its type detected to date: it is 10 times brighter than the previous record holder. In one second it emits the same amount of energy released by our Sun in 3.5 years.

- XMM-Newton observed the object several times in the last 13 years, with the discovery a result of a systematic search for pulsars in the data archive – its 1.13 s periodic pulses giving it away. The signal was also identified in NASA's NuSTAR archive data, providing additional information.

- "Before, it was believed that only black holes at least 10 times more massive than our Sun feeding off their stellar companions, could achieve such extraordinary luminosities, but the rapid and regular pulsations of this source are the fingerprints of neutron stars and clearly distinguish them from black holes," says Gian Luca Israel, from the INAF-Osservatorio Astronomica di Roma, Italy, lead author of the paper describing the result published in Science this week.

- The archival data also revealed that the pulsar's spin rate has changed over time, from 1.43 s per rotation in 2003 to 1.13 s in 2014. The same relative acceleration in Earth's rotation would shorten a day by five hours in the same time span. "Only a neutron star is compact enough to keep itself together while rotating so fast," adds Gian Luca.

- Although it is not unusual for the rotation rate of a neutron star to change, the high rate of change in this case is likely linked to the object rapidly consuming mass from a companion. "This object is really challenging our current understanding of the ‘accretion' process for high-luminosity stars," says Gian Luca. "It is 1000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object."

- The scientists think there must be a strong, complex magnetic field close to its surface, such that accretion onto the neutron star surface is still possible while still generating the high luminosity.

- "The discovery of this very unusual object, by far the most extreme ever discovered in terms of distance, luminosity and rate of increase of its rotation frequency, sets a new record for XMM-Newton, and is changing our ideas of how such objects really ‘work'," says Norbert Schartel, ESA's XMM-Newton project scientist.

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Figure 34: The record-breaking pulsar, identified as NGC 5907 X-1, is in the spiral galaxy NGC 5907, which is also known as the Knife Edge Galaxy or Splinter Galaxy. The image comprises X-ray emission data (blue/white) from ESA's XMM-Newton space telescope and NASA's Chandra X-ray observatory, and optical data from the Sloan Digital Sky Survey (galaxy and foreground stars), image credit: ESA/XMM-Newton; NASA/Chandra and SDSS

Legend to Figure 34: The inset shows the X-ray pulsation of the spinning neutron star, which has a period of 1.13 s, as determined by XMM-Newton's European Photon Imaging Camera.

• January 31, 2017: Scientists observing a curious neutron star in a binary system known as the 'Rapid Burster' may have solved a forty-year-old mystery surrounding its puzzling X-ray bursts. They discovered that its magnetic field creates a gap around the star, largely preventing it from feeding on matter from its stellar companion. Gas builds up until, under certain conditions, it hits the neutron star all at once, producing intense flashes of X-rays. The discovery was made with space telescopes including ESA's XMM-Newton. 36)

- Discovered in the 1970s, the Rapid Burster is a binary system comprising a low-mass star in its prime and a neutron star – the compact remnant of a massive star's demise. In such a stellar pair, the gravitational pull of the dense remnant strips the other star of some of its gas; the gas forms an accretion disc and spirals towards the neutron star.

- As a result of this accretion process, most neutron star binaries continuously release large amounts of X-rays, which are punctuated by additional X-ray flashes every few hours or days. Scientists can account for these 'type-I' bursts, in terms of nuclear reactions that are ignited in the inflowing gas – mainly hydrogen – when it accumulates on the neutron star's surface.

- But the Rapid Burster is a peculiar source: at its brightest, it does emit these type-I flashes, while during periods of lower X-ray emission, it exhibits the much more elusive 'type-II' bursts – these are sudden, erratic and extremely intense releases of X-rays.

- In contrast to type-I bursts, which do not represent a significant release of energy with respect to what is normally emitted by the accreting neutron star, bursts of type-II liberate enormous amounts of energy during periods otherwise characterized by very little emission occurring (the relative energy output of a burst with respect to the normal accretion process is tens to hundreds of times higher in type-II bursts than in type-I bursts).

- Despite forty years of searches, type-II bursts have been detected only in one other source besides the Rapid Burster. Known as the Bursting Pulsar and discovered in the 1990s, this binary system comprises a low-mass star and a highly magnetized, spinning neutron star – a pulsar – that exhibits only type-II bursts.

- Because of the scarcity of sources that display this phenomenon, the underlying physical mechanisms have long been debated, but a new study of the Rapid Burster provides first evidence for what is occurring.

- "The Rapid Burster is the archetypal system to investigate type-II bursts – it's where they were first observed and the only source that shows both type-I and type-II bursts," says Jakob van den Eijnden, a PhD student at the Anton Pannekoek Institute for Astronomy in Amsterdam, The Netherlands, and lead author of a Letter published in Monthly Notices of the Royal Astronomical Society. 37) — In this study, Jakob and his colleagues organized an observing campaign using three X-ray space telescopes to find out more about this system.

- Under the coordination of co-author Tullio Bagnoli, who was also based at the Anton Pannekoek Institute for Astronomy, the team managed to observe the source bursting over a few days in October 2015 with a combination of NASA's NuSTAR and Swift, and ESA's XMM-Newton.

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Figure 35: Variations of brightness observed in the binary system MXB 1730-335, also known as the 'Rapid Burster' as observed by NASA's NuSTAR X-ray telescope (image credit: NASA, adapted from Ref. 37) 38)

- They first monitored the source with Swift, timing the observations for a period when they expected a series of type-II bursts to take place. Then, soon after the first burst was detected, the scientists set the other observatories into motion, using XMM-Newton to measure X-rays emitted directly by the neutron star's surface or by gas in the accretion disc, and NuSTAR to detect higher-energy X-rays, which are emitted by the neutron star and reflected off the disc. - With these data, the scientists scrutinized the structure of the accretion disc to understand what happens to it before, during, and after these copious releases of X-rays.

- According to one model, type-II bursts occur because the fast spinning magnetic field of the neutron star keeps the gas flowing from the companion star at bay, preventing it from reaching closer to the neutron star and effectively creating an inner edge at the center of the disc. However, as the gas continues to flow and accumulate near this edge, it spins faster and faster, and eventually catches up with the spinning velocity of the magnetic field.

- "It's as if we threw something towards a merry-go-round that is spinning very fast: it would bounce off, unless it's thrown at the same velocity as the machine," explains Jakob. "A similar balancing act happens between the inflowing gas and the spinning magnetic field: as long as the gas hasn't the right speed, it cannot get to the neutron star and it can only pile up at the edge. By the time it reaches the right velocity, a lot of gas has accumulated and it hits the neutron star all at once, giving rise to the dramatic emission of type-II bursts."

- This model predicts that, while the material is piling up, a gap should form between the neutron star and the edge of the accretion disc.

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Figure 36: Artist's impression of the neutron star in the Rapid Burster (image credit: ESA/ATG medialab)

- In other models, the intense flashes are explained as arising from instabilities in the flow of the accreting gas or from general-relativistic effects. In either case, these would take place much closer to the neutron star and not give rise to such a gap. "A gap is exactly what we found at the Rapid Burster," says Nathalie Degenaar, a researcher at Anton Pannekoek Institute for Astronomy and Jakob's PhD advisor. "This strongly suggests that the type-II bursts are caused by the magnetic field."

- The observations indicate that there is a gap of roughly 90 km between the neutron star and the inner edge of the accretion disc. While not impressive on cosmic scales, the size of the gap is much larger than the neutron star itself, which has a radius of about 10 km.

- This finding is in line with results from a previous study by Nathalie and collaborators, who had observed a similar gap around the Bursting Pulsar – the other source known to produce type-II bursts.

- In the new study of the Rapid Burster, the scientists also measured the strength of the neutron star's magnetic field: at 6 x 108 G (gauss), it is around a billion times stronger than Earth's and, most important, over five times stronger than observed in other neutron stars with a low-mass stellar companion. This could hint at a young age of this binary system, suggesting that the accretion process has not been going on for long enough to damp the magnetic field down, as is thought to have happened in similar systems.

- If this neutron star binary really is as young as its strong magnetic field indicates, then it is expected to spin much slower than its older counterparts: future measurements of the star's spinning rate might help confirm this unusual scenario.

- "This result is a big step towards solving a forty-year-old puzzle in neutron star astronomy, while also revealing new details about the interaction between magnetic fields and accretion discs in these exotic objects," concludes Norbert Schartel, XMM-Newton Project Scientist at ESA.

• December 19, 2016: An important part of studying celestial objects is understanding and removing the background noise. The image presented in Figure 37 was created to demonstrate the power of software tools used to analyze observations by ESA's XMM-Newton of large objects like galaxies, clusters of galaxies, and supernova remnants. The tool models and subtracts the background noise, which is very difficult for large, fuzzy objects like these, and to create exposure-corrected images. It also merges and smooths the observations taken of an individual object by XMM-Newton's three X-ray cameras of the EPIC (European Photon Imaging Camera) instrument, and to allow the mosaicking of multiple observations. 39)

- It certainly does not look like it, but the star-like feature on the right corresponds to spiral galaxy M81. Similarly, the feature on the left arises from the Holmberg IX dwarf galaxy.

- By examining images like these, along with complementary images taken at other wavelengths, scientists can get a quick look at the structure of the object and the spectral variations across the field. The structure of the bright patterns contains information about the origin of the emission, such as whether it comes from a ‘halo' around the galaxy, or is confined to the disc and arms.

- For example, this particular image shows that there is a bright point source at the center of M81, resulting from the galaxy's active core. There is also a decrease of brightness away from the central source, with fainter extended emission around it. Another galaxy might display brighter emission along its spiral arms.

- Bright emission is also apparent from the X-ray source in the dwarf galaxy. The ‘rays' extending from the point sources are artefacts, seen whenever there is a very bright point source in the field of view. But even artefacts can be beautiful.

- The XMM-Newton Extended Source Analysis Software package (XMM-ESAS) was developed at the NASA/GSFC ( Goddard Space Flight Center) XMM-Newton GOF (Guest Observer Facility) in cooperation with the XMM-Newton Science Operations Center and the Background Working Group.

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Figure 37: A familiar galaxy pair takes on an unusual appearance with bright points and delicate rays (image credit: ESA/XMM-Newton)

• December 8, 2016: XMM-Newton is one of Europe's longest-flying and most productive orbiting observatories, investigating the hot X-ray Universe. Thanks to teamwork and technical innovation, it's on track to keep flying for a long time yet. Launched 17 years ago, ESA's orbiting X-ray telescope has helped scientists around the world to understand some of our Universe's most mysterious events, from what happens in and around black holes to how galaxies formed. At 3800 kg, the 10 m-long XMM-Newton is the biggest science satellite ever built in Europe and its telescope mirrors are the most sensitive ever developed.-Expected to operate for as long as a decade, the hardy spacecraft has happily surprised everyone by lasting almost two decades – and it shows no signs of giving up. 40)

- The success of XMM-Newton has been possible not only because of the robust spacecraft, but also the close cooperation between ESA's astronomy center near Madrid, Spain, and the mission controllers at ESA's operations center in Darmstadt, Germany.

- "The total number of 4775 scientific publications to date, with 358 from this year alone, is an impressive record of the mission's scientific success, covering many, many areas of astrophysics," notes project scientist Norbert Schartel.

- But keeping it fit and healthy into its third decade means the team must continue to develop and test new control techniques. A complex change to the orbit control system has almost halved fuel consumption, for example.

- (Not) running on empty: For starters, keeping XMM in orbit will require occasional thruster firings, about once per day, and that means burning fuel. "We've got plenty of fuel and over the years we've figured out how to use less and less to maintain our science orbit," says Marcus Kirsch, spacecraft operations manager. "The fuel is distributed between four separate tanks, but the main tank will run dry first. The design means we could not use the remaining fuel in the other tanks, so we're moving it all into tank 1. This will enable us to continue operations into the coming decade."

- Back in the control room: As part of this process, the flight control team returned to ESA's large, general-purpose Main Control Room at mission control in November – the first time since launch in 1999 – for five days of intensive simulations. The team usually works from a smaller, dedicated room shared with the Integral and Gaia mission teams. — The simulations checked the procedures that will be used for moving the fuel and for reconfiguring XMM for working beyond 2017.

- No one's ever done this before: "Not many spacecraft use the specially designed tank fuel system like on XMM," says Nikolai von Krusenstiern, spacecraft operations engineer. - "As far as we know, no one's ever shifted fuel from one tank to another with a tank design like ours in a satellite in orbit, and we want to take as much time as necessary to minimize any risk to the mission." Tank-to-tank replenishment was never foreseen in the original design specifications – as XMM wasn't meant to last so long – so no process was devised by builder Astrium (now Airbus Defence & Space). "Airbus have been very helpful – they even helped us get in touch with the now-retired designer of the fuel system to help us to safely design the procedures," says Nikolai.

- XMM's third decade: The team will now analyze results of last month's simulations with the aim of reconfiguring the spacecraft in 2017. This will complement the careful optimization of the flight control procedures already in place, and keep XMM thrusters firing – and the spacecraft flying reliably – into 2023. After that, the team will have a low-risk and confirmed plan on hand to conduct the fuel replenishment, which would thereafter keep the craft in its science mission well into its third decade. "The time spent in training and simulations last month was hugely valuable for the entire team," says Marcus. "We worked together to devise a solid solution for XMM's coming decades, and the individual engineers gained excellent training experience that they can use for XMM or even take with them if assigned to other missions."

• December 8, 2016: XMM-Newton delivers a selfie to mark 17 years of X-ray science success. Launched on 10 December 1999, XMM-Newton is an X-ray observatory designed to investigate some of the most violent phenomena in the Universe. Sources that emit large amounts of X-rays include remnants of supernova explosions and the surroundings of black holes. — Detecting this energetic radiation is a daunting endeavor, requiring techniques that are greatly different from those used in traditional telescopes. In the case of XMM-Newton, it carries three telescopes of 58 nested mirrors each. These sit at one end of a 7 m-long tube, while at the other end are the scientific instruments at the focus. 41)

- The cameras were originally used by controllers to check how the solar wings unfolded after launch, and have remained dormant since 2003.

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Figure 38: The two images in this collage were taken by the two low-resolution monitoring cameras mounted on opposite sides of the focal plane assembly, looking along the pointing direction of the telescope tube towards the service module (see Figure 39 for an annotated version with explanation). When these images were captured on 14 September 2016 at 06:50 GMT, XMM-Newton was in its 3070th orbit at around 50,000 km altitude and in contact with mission controllers at ESA's mission control in Darmstadt, Germany, via the antenna at Kourou, French Guiana.

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Figure 39: Image explanation if Figure 38 (image credit: ESA)

- In the image on the left, one camera captured the Sun side of one of XMM's solar wings (at left in the image), and the dark multilayer insulation on the service module, the bright Sun-shielding behind and a dark box-like structure topped by a pair of thrusters (at right in the image).

- In the image on the right, the other camera captured the dark tripod of the S-band antenna (at left in the image) and then the 2A/2B thruster pair (at center) and XMM's other solar wing (at right).

• November 22, 2016: ESA's Science Program Committee (SPC) has today confirmed two-year mission extensions for nine scientific missions in which the Agency is participating. This secures their operations until the end of 2018. — After a comprehensive review of their current operational status and the likely scientific return from each mission, the SPC decided to extend the operation of six ESA-led missions (Cluster, INTEGRAL, Mars Express, PROBA-2, SOHO and XMM-Newton) from 1 January 2017 to 31 December 2018. 42)

- The go-ahead was also given to continue ESA's contributions to the operations of three international collaborative missions: the Hubble Space Telescope and the Interface Region Imaging Spectrograph (IRIS), which are both led by NASA, as well as Solar-B (Hinode), which is a Japanese-led mission.

• August 29, 2016: A giant bubble surrounding the center of the Milky Way shows that six million years ago our Galaxy's supermassive black hole was ablaze with furious energy. It also shines a light on the hiding place of the Galaxy's so-called 'missing' matter. - While the mysterious dark matter grabs most of the headlines, astronomers also know that they have yet to find all of the normal, so-called baryonic, matter in the Galaxy. That has now changed thanks to the work of ESA's X-ray observatory XMM-Newton. 43)

- A thorough analysis of archival observations has shown that there is a vast quantity of baryonic matter scattered through the Galaxy. XMM-Newton found it in the form of gas at a temperature of one million degrees that permeates both the disc of the Galaxy, where the majority of the stars are found, and a spherical volume that surrounds the whole Galaxy. — The spherical cloud is vast. Whereas the Sun lies just 26 000 light years from the center of the Galaxy, the cloud extends out to a distance of at least 200 000 – 650 000 light years.

- Fabrizio Nicastro, from the Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma, Italy, and his colleagues have been on the trail of the missing baryons for more than 15 years now. Their latest discovery with XMM-Newton shows that there is enough million-degree-hot gas in the Galaxy to account for it all. 44)

- It has remained undetected for so long because it does not emit visible light. Instead, the astronomers found it because the oxygen in the cloud absorbed X-rays at very specific wavelengths from light being emitted by more distant celestial objects. - And this was not the only discovery waiting in the data for the team. When it came to model the data with computer simulations to understand the way in which the gas was distributed around the Galaxy, the team did not get the answer they were expecting.

- "According to simple gravitational physics you expect the density of the gas to decrease from the center outwards," says Nicastro. In this picture, the density of gas will be at its peak in the center of the Galaxy and at its least on the outer edges. But there was a hitch. "I spent three months trying to match the data with that model and I just couldn't," says Nicastro.

- Having tried everything else, he moved the peak density away from the center of the Galaxy. At a distance of about 20 000 light years from the Galaxy's center the model fitted better. It was puzzling why this should improve things until he remembered that this distance is also the size of two large 'balloons' of gamma rays found in 2010 by NASA's Fermi gamma-ray observatory, which extend tens of thousands of light-years above and below the center of our Galaxy.

- So Nicastro constructed a different density model, in which there was a central bubble of low density gas extending out to 20 000 light years. When he applied this model to his X-ray data, he found that it fitted excellently. "That was unexpected and very exciting at the same time," says Nicastro. It meant that something had pushed the gas outwards from the center of the Galaxy, creating a giant bubble.

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Figure 40: This artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity. A wispy orange bubble extends from the galactic center out to a radius of about 20,000 light-years. Outside of that bubble, a pervasive "fog" of million-degree gas might account for the Galaxy's missing matter of 130 billion solar masses (image credit: Mark A. Garlick/CfA) 45)

• July 12, 2016: ESA's orbiting X-ray observatory, XMM-Newton, has proved the existence of a 'gravitational vortex' around a black hole. The discovery, aided by NASA's NuSTAR mission, solves a mystery that has eluded astronomers for more than 30 years and will allow them to map the behavior of matter very close to black holes. It could also open the door to future investigations of Albert Einstein's general relativity. 46) 47) 48)

- Matter falling into a black hole heats up as it plunges to its doom. Before it passes into the black hole and is lost from view forever, it can reach millions of degrees. At that temperature it shines X-rays into space.

- In the 1980s, pioneering astronomers using early X-ray telescopes discovered that the X-rays coming from black holes flicker. The changes follow a set pattern. When the flickering begins, the dimming and re-brightening can take 10 seconds to complete. As the days, weeks and then months progress, the period shortens until the oscillation takes place 10 times every second. Then, the flickering suddenly stops altogether.

- The phenomenon was dubbed the QPO (Quasi Periodic Oscillation). "It was immediately recognized to be something fascinating because it is coming from something very close to a black hole," says Adam Ingram, University of Amsterdam, The Netherlands, who began working to understand QPOs for his PhD in 2009. — During the 1990s, astronomers had begun to suspect that the QPOs were associated with a gravitational effect predicted by Einstein's general relativity: that a spinning object will create a kind of gravitational vortex.

- "It is a bit like twisting a spoon in honey. Imagine that the honey is space and anything embedded in the honey will be 'dragged' around by the twisting spoon," explains Ingram. "In reality, this means that anything orbiting a spinning object will have its motion affected." In the case of an inclined orbit, it will 'precess'. This means that the whole orbit will change orientation around the central object. The time for the orbit to return to its initial condition is known as a precession cycle.

- In 2004, NASA launched Gravity Probe B to measure this so-called Lense-Thirring effect around Earth. After painstaking analysis, scientists confirmed that the spacecraft would turn through a complete precession cycle once every 33 million years.

- Around a black hole, however, the effect would be much more noticeable because of the stronger gravitational field. The precession cycle would take just a matter of seconds or less to complete. This is so close to the periods of the QPOs that astronomers began to suspect a link.

- Ingram began working on the problem during his PhD, looking at what happened in the flat disc of matter surrounding a black hole. Known as an accretion disc, it is the place where material gradually spirals inwards towards the black hole. It had already been suggested that, close to the black hole, the flat accretion disc puffs up into a hot plasma, in which electrons are stripped from their host atoms. Termed the hot inner flow, it shrinks in size over weeks and months as it is eaten by the black hole. Together with colleagues, Ingram published a paper in 2009 suggesting that the QPO is driven by Lense-Thirring precession of this hot flow. This is because the smaller the inner flow becomes, the closer to the black hole it would approach and so the faster its Lense-Thirring precession cycle would be. The question was: how to prove it?

- The answer was that the inner flow is releasing high energy radiation that strikes the matter in the surrounding accretion disc, making the iron atoms in the disc shine like a fluorescent light tube. Instead of visible light, the iron releases X-rays of a single wavelength - referred to as 'a line'.

- Because the accretion disc is rotating, the iron line has its wavelength distorted by the Doppler effect. Line emission from the approaching side of the disc is squashed – blue shifted – and line emission from the receding disc material is stretched – red shifted. If the inner flow really is precessing, it will sometimes shine on the approaching disc material and sometimes on the receding material, making the line wobble back and forth over the course of a precession cycle.

- Seeing this wobbling is where XMM-Newton came in. Ingram and colleagues from Amsterdam, Cambridge, Durham, Southampton and Tokyo applied for a long duration observation that would allow them to watch the QPO repeatedly. They chose black hole H 1743-322, which was exhibiting a four-second QPO at the time. They watched it for 260,000 seconds with XMM-Newton. They also observed it for 70,000 seconds with NASA's NuSTAR X-ray observatory.

- After a complicated analysis procedure to add all the observational data together, they saw that the iron line was wobbling in accordance with the predictions of general relativity. "We are directly measuring the motion of matter in a strong gravitational field near to a black hole," says Ingram.

- This is the first time that the Lense-Thirring effect has been measured in a strong gravitational field. The technique will allow astronomers to map matter in the inner regions of accretion discs around black holes. It also hints at a powerful new tool with which to test general relativity.

- Einstein's theory is largely untested in such strong gravitational fields. So if astronomers can understand the physics of the matter that is flowing into the black hole, they can use it to test the predictions of general relativity as never before - but only if the movement of the matter in the accretion disc can be completely understood.

- "This is a major breakthrough since the study combines information about the timing and energy of X-ray photons to settle the 30-year debate around the origin of QPOs. The photon collecting capability of XMM-Newton was instrumental in this work," says Norbert Schartel, ESA Project Scientist for XMM-Newton.

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Figure 41: This artist's impression depicts the accretion disc surrounding a black hole, in which the inner region of the disc precesses (image credit: ESA/ATG medialab)

Legend to Figure 41: In these three views, the precessing inner disc shines high energy radiation that strikes the matter in the surrounding accretion disc, causing the iron atoms in that disc to emit in X-rays, depicted as the glow on the accretion disc to the right (in view a), to the front (in view b) and to the left (in view c).

• May 2016: Current technical evaluations suggest that the XMM-Newton spacecraft and its scientific instruments may continue to provide first class X-ray observations well into the next decade. 49)

May 2016: Improvement of the operational XMM-Newton system for more efficient operations support. The original design of the XMM-Newton mission operations was based on continuous ground station coverage and cannot be changed anymore, since the spacecraft has no mass memory on board. Telecommanding and telemetry reception is therefore handled "live" from the MCS (Mission Control System) with individual commands, sent for routine operations prepared and bundled into an automated timeline. This timeline runs in real time and sends all the required commands. Verification of telecommanding is done by the SPACON (SPAcecraft CONtroller), using SCOS-2000 (Spacecraft Control & Operation System-2000) as mission control system. 50)

- Through an operational change of the AOCS system, the project team extended its potential lifetime until the late 20's of this century. Spacecraft and instruments are performing without major degradation. Because of the highly elliptical orbit, the spacecraft is permanently visible from the ground station network. For this and other reasons, the originally chosen on-board autonomy design is very limited. Fifteen years of inflight experience created a deep understanding of the spacecraft behavior including elaborated strategies and procedures to recover from reoccurring non-critical anomalies on board. Based on this experience, an automated ground failure detection system has been developed to provide reliable fault isolation and adapted reaction.

- The newly implemented on-ground FDIR (Failure Detection, Isolation and Recovery) system is able to read various sorts of anomaly messages, either from the spacecraft (telemetry being out-of-limit), from the ground segment (link loss) or from the mission control software (an application stopped). Upon detection of anomaly, the automation system starts the execution of a procedure to confirm its consistency and persistence to avoid over reaction to minor events. Once the anomaly has been confirmed and identified, the recovery is kicked off under the supervision of the operator or autonomously. The targeted level of automation shall release the operator from activities that were so far manually performed, let him concentrate on other activities running in parallel, reduce the stress in contingency situations and lessen the probability of human error.

- Outlook: The usage of an automation system has shown its relevance on a mission like XMM-Newton so that the flight control team members are stepping forward to include more automated procedures. Among the upcoming challenges, the automation of the ground stations activities is quite promising. Indeed, the spacecraft controller has to coordinate via the voice loop with the operator of ESA's tracking station network. Being able to control the setup of the TM and TC links would contribute to largely increase the autonomy of operations performed from ground.

- The on-ground FDIR system, called MOIS (Manufacturing and Operation Information System) Event Monitor, has been designed to react to different types of events. The detection relies on collection of known events and results in the trigger of an automated procedure. This procedure checks the consistency of the anomaly using details provided by the mission control system and a MOIS internal database. Assuming that the anomaly is confirmed, the links to the satellite are correctly established and there is no constraining up-coming activity, MOIS requests the start of the contingency recovery procedure. Belonging to a larger automation system, the Event Monitor is a real support for the human controller and facilitates his work during repeating and heavy operations.

- Every contingency recovery procedures that had to be automated was reviewed and streamlined. After such a long time in orbit, this was a good exercise for the flight control team to re-familiarize with the procedures and also a great opportunity to update and increase further the robustness of the procedures. The automation system is now operational and pleads for another decade of safe operations with XMM-Newton.

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Figure 42: Overview of the Automation system implementation within the execution environment. In addition to the automated planned activities, the Event Monitor triggers the execution of recovery upon notification of anomaly from the mission control system (image credit: ESA)

• May 2016: With 140 kg of propellant left onboard after the orbit injection maneuver, for a planned 10 years lifetime (elapsed in 2009) using approximately 6 kg propellant per year, propellant seemed not to be a life limiting factor for the mission, which scientific goal is to unveil the hot and violent universe with an unprecedented accuracy in terms of spectral and imaging detection capabilities. However the success of XMM-Newton reflected in the very high demand of observation time requested and the scientific output generated by the worldwide X-ray community turned this initially almost neglected factor into a major one, since the mission was extended various times because spacecraft and instruments are still operating without major degradation reducing the life limiting factors to the fuel reserve only. Measures to significantly reduce fuel consumption were already taken. Next, the Main Tank replenishing procedure had to be further developed, and in-flight tests were needed to determine performance and performance limits. Up to now, the RCS operated reliably, requiring only a minimum of attention. As long as there is enough propellant to keep all 4 tanks in wet conditions, the precise performance of the tank heater system, the accuracy of the 8 thermistors (2 on each tank), and the accurate knowledge of the gas temperature inside each tank were quite irrelevant issues. This is now radically changing with Main Tank depletion coming closer and with the need to develop, validate and implement the Main Tank replenishing procedure.51)

- Because propellant migration and Main Tank replenishing can only be achieved by temperature changes, virtually all functions of the tank heater system and the thermal response of the individual tanks had to be tested and characterized by various in-flight tests. Taking into account the findings and experience gained by these in-flight tests and observing various constraints, a feasible Main Tank replenishing procedure was developed, which has enough performance margin to be called a "robust procedure". The main parts of the procedure are already validated.

- Outlook: The large propellant gaging uncertainty is a very unsatisfactory. Attempts to reduce these uncertainties will continue in 2016. With the Main Tank replenishing procedure established, various mission planning and optimization exercises can be made, and a suitable planning tool to support operations later on should preserve the know-how gained.

• April 28, 2016: ESA's XMM-Newton has discovered gas streaming away at a quarter of the speed of light from very bright X-ray binaries in two nearby galaxies. At X-ray wavelengths, the celestial sky is dominated by two types of astronomical objects: supermassive black holes, sitting at the centers of large galaxies and ferociously devouring the material around them, and binary systems, consisting of a stellar remnant – a white dwarf, neutron star or black hole – feeding on gas from a companion star. 52) 53)

- In both cases, the gas forms a swirling disc around the compact and very dense central object: friction in the disc causes the gas to heat up and emit light at many wavelengths, with a peak in X-rays. Not all of the gas is swallowed by the central object though, and some of it might even be pushed away by powerful winds and jets.

- But an intermediate class of objects was discovered in the 1980s and is still not well understood. Ten to a hundred times brighter than ordinary X-ray binaries, these sources are nevertheless too faint to be linked to accreting supermassive black holes, and in any case, are usually found far from the center of their host galaxy. "We think these 'ultra-luminous X-ray sources' are somewhat special binary systems, sucking up gas at a much higher rate than an ordinary X-ray binary," explains Ciro Pinto from the Institute of Astronomy in Cambridge, UK. "Some host highly magnetized neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around 1000 times the mass of the Sun. But in the majority of cases, the reason for their extreme behavior is still unclear."

- Ciro and his colleagues delved into the XMM-Newton archives and collected several days' worth of observations of three ultra-luminous X-ray sources, all hosted in nearby galaxies located less than 22 million light-years from our Milky Way. The data were obtained over several years with the RGS (Reflection Grating Spectrometer), a highly sensitive instrument that allowed them to spot very subtle features in the spectrum of the X-rays from the sources.

- In all three sources, the scientists were able to identify X-ray emission from gas in the outer portions of the disc surrounding the central compact object, slowly flowing towards it. But two of the three sources – known as NGC 1313 X-1 and NGC 5408 X-1 – also show clear signs of X-rays being absorbed by gas that is streaming away from the central source at an extremely rapid 70 ,000 km/s – almost a quarter of the speed of light.

- "This is the first time we've seen winds streaming away from ultra-luminous X-ray sources," says Ciro. "And there's more, since the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter."

- While the hot gas is pulled inwards by the central object's gravity, it also shines brightly, and the pressure exerted by the radiation pushes it outwards. This is a balancing act: the greater the mass, the faster it draws the surrounding gas. But this also causes the gas to heat up faster, emitting more light and increasing the pressure that blows the gas away.

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Figure 43: Artist's impression depicting a compact object – either a black hole or a neutron star – feeding on gas from a companion star in a binary system (image credit: ESA, C. Carreau)

Legend to Figure 43: Since gas cannot fall in from all directions in a rotating system, it forms a swirling disc around the compact object. This causes matter to heat up and emit light at many wavelengths, especially X-rays. However, not all the gas in the disc is swallowed, and some of it is blown away in the form of winds or jets. - Scientists using ESA's XMM-Newton have discovered gas streaming away at a quarter of the speed of light from two very bright X-ray binaries, known as ultra-luminous X-ray sources, that are located in nearby galaxies. The discovery confirms that these sources conceal a compact object accreting matter at extraordinarily high rates.

• March 31, 2016: Decades of searching in the Milky Way's nearby ‘twin' galaxy Andromeda have finally paid off, with the discovery of an elusive breed of stellar corpse, a neutron star, by ESA's XMM-Newton space telescope. Andromeda, or M31, is a popular target among astronomers. Under clear, dark skies it is even visible to the naked eye. Its proximity and similarity in structure to our own spiral galaxy, the Milky Way, make it an important natural laboratory for astronomers. It has been extensively studied for decades by telescopes covering the whole electromagnetic spectrum. 54) 55)

- Despite being extremely well studied, one particular class of object had never been detected: spinning neutron stars. Neutron stars are the small and extraordinarily dense remains of a once-massive star that exploded as a powerful supernova at the end of its natural life. They often spin very rapidly and can sweep regular pulses of radiation towards Earth, like a lighthouse beacon appearing to flash on and off as it rotates.

- These ‘pulsars' can be found in stellar couples, with the neutron star cannibalizing its neighbor. This can lead to the neutron star spinning faster, and to pulses of high-energy X-rays from hot gas being funnelled down magnetic fields on to the neutron star.

- Binary systems hosting a neutron star like this are quite common in our own Galaxy, but regular signals from such a pairing had never before been seen in Andromeda.

- Now, astronomers systematically searching through the archives of data from XMM-Newton X-ray telescope have uncovered the signal of an unusual source fitting the bill of a fast-spinning neutron star. It spins every 1.2 seconds, and appears to be feeding on a neighboring star that orbits it every 1.3 days.

- "We were expecting to detect periodic signals among the brightest X-ray objects in Andromeda, in line with what we already found during the 1960s and 1970s in our own Galaxy," says Gian Luca Israel, from INAF-Osservatorio Astronomica di Roma, Italy, one of the authors of the paper describing the results, "But persistent, bright X-ray pulsars like this are still somewhat peculiar, so it was not completely a sure thing we would find one in Andromeda. "We looked through archival data of Andromeda spanning 2000–13, but it wasn't until 2015 that we were finally able to identify this object in the galaxy's outer spiral in just two of the 35 measurements."

- While the precise nature of the system remains unclear, the data imply that it is unusual and exotic. "It could be what we call a ‘peculiar low-mass X-ray binary pulsar' – in which the companion star is less massive than our Sun – or alternatively an intermediate-mass binary system, with a companion of about two solar masses," says Paolo Esposito of INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, Italy.

- "We need to acquire more observations of the pulsar and its companion to help determine which scenario is more likely."

- "The well-known Andromeda galaxy has long been a source of exciting discoveries, and now an intriguing periodic signal has been detected by our flagship X-ray mission," adds Norbert Schartel, ESA's XMM-Newton project scientist. "We're in a better position now to uncover more objects like this in Andromeda, both with XMM-Newton and with future missions such as ESA's next-generation high-energy observatory, Athena."

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Figure 44: Andromeda, or M31, is a spiral galaxy similar to our own Milky Way. For the first time, a spinning neutron star has been inferred in XMM-Newton data [image credit: Andromeda: ESA/Herschel/PACS/SPIRE, J. Fritz, U. Gent, XMM-Newton/EPIC, W. Pietsch, MPE; data: P. Esposito et al., (2016)]

Legend to Figure 44: Inset: Light curve of the source, known as 3XMM J004301.4+413017, as analyzed by XMM-Newton's European Photon Imaging Camera, EPIC. The source has a period of 1.2 seconds, consistent with a spinning neutron star.

• December 15, 2015: Today marks the release of the first papers to result from the XXL survey, the largest survey of galaxy clusters ever undertaken with ESA's XMM-Newton X-ray observatory. The gargantuan clusters of galaxies surveyed are key features of the large-scale structure of the Universe and to better understand them is to better understand this structure and the circumstances that led to its evolution. The first results from the survey, published in a special issue of Astronomy and Astrophysics, hint at the answers and surprises that are captured in this unique bank of data and reveal the true potential of the survey. 56)

- The XXL project, the largest XMM-Newton observing program to date, set itself the ambitious task of mapping galaxy clusters back to a time when the Universe was just half of its present age. Its aim was to trace the evolution of the large-scale structure of the Universe. When looking at the large-scale distribution of matter in the Universe we see that it is not evenly distributed throughout space but forms a cosmic web of matter-filled filaments and a network of vast matter-less voids. Within these filaments huge swathes of matter have been pulled together by the effect of gravity into galaxy clusters, the largest bound entities in the Universe. Studying how these clusters evolve over time, in surveys like XXL, holds the key to uncovering how the arrangement of matter in the Universe has changed – the dense getting denser and the voids becoming emptier – and gives valuable insights into the accelerated expansion of the Universe and the factors that drive it.

- "Tracing the evolution of cosmic structure is of special interest to astronomers," explains Marguerite Pierre, principal investigator of the XXL project. "How the structure has evolved depends on certain important factors like the density of the Universe and the acceleration of its expansion. The better our knowledge of how this structure has evolved, the more accurate our models of the Universe become. This in turn helps us to dig deeper into the nature of the most mysterious components of our Universe – dark matter and dark energy."

- The XXL survey covered two huge regions each of which, if viewed on the sky, would have a two-dimensional area a hundred times larger than the full Moon, and that is without taking into account the depth that the survey explored. These already extensive swathes of sky were mapped back to a time when the Universe was half of its present age with a total of 450 galaxy clusters detected, and their properties and distributions mapped, in the process. This is no mean feat as, for many clusters, fewer X-ray photons were detected per cluster than there are galaxies in that cluster. Despite this meager amount of information the team was still able to identify clusters much more easily than would be possible in visible or infrared light by detecting the huge reservoirs of gas that fill the space between the resident galaxies, rather than trying to pick out the light emitted by the visible matter in the clusters. These gas reservoirs are heated to a few tens of millions of degrees and can be detected as X-ray emission.

- Today, the XXL team present their first results in a set of thirteen papers published in a special issue of Astronomy and Astrophysics. Among these initial findings, which are based on a catalog of the 100 brightest clusters in the survey – also released today – the promise of shedding new light on dark energy and matter is already being met, with evidence emerging that could lead to refinements of current cosmological models. 57)

- Prior to the survey the XXL team made predictions about the number of clusters they expected to find, and how densely they expected them to be spread across the sky, by using data from ESA's Planck telescope, which measures the remnant radiation from the Big Bang. To do this requires careful calculation of how the Universe has expanded since its inception, a phenomenon driven by the energy content of the Universe. When the team analyzed the data from XXL they found that the density of clusters was in fact lower than had been predicted by modelling using Planck data. Although the reason for the discrepancy remains unknown, it could imply that our Universe is somewhat different or more complex than the current cosmological model favored by Planck or, alternatively, that our understanding of the physics and evolution of galaxy clusters needs to be improved. Further investigation will be carried out with the full sample of clusters in 2017.

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Figure 45: This image shows the XXL-South field (or XXL-S), one of the two fields observed by the XXL survey (image credit: ESA)

Legend to Figure 45: XXL is the largest survey of galaxy clusters ever undertaken and provides by far the best view of the deep X-ray sky yet obtained. The survey was carried out with ESA's XMM-Newton X-ray observatory. The area shown in this image was obtained with some 220 XMM-Newton pointings and, if viewed on the sky, would have a two-dimensional area a hundred times larger than the full Moon (which spans one half degree), and that is without taking into account the depth that the survey explores. The red circles in this image show the clusters of galaxies detected in the survey. Along with the other field – XXL-North field (or XXL-N) – around 450 of these clusters were uncovered in the survey, which mapped them back to a time when the Universe was just half of its present age. The image also reveals some of the 12 000 Active Galactic Nuclei that were detected in the field.

• December 3, 2015: Scientists have developed a technique to use quasars – powerful sources driven by supermassive black holes at the center of galaxies – to study the Universe's history and composition. To demonstrate the new method, based on a relation between a quasar's luminosity at X-ray and ultraviolet wavelengths, they made extensive use of data from ESA's XMM-Newton X-ray observatory. This approach promises to become an important tool to constrain the properties of our Universe. 58)

- At the core of most massive galaxies in the Universe is a supermassive black hole – a concentration of matter so dense that it attracts anything nearby, including light. Such black holes have masses from millions to billions of times that of the Sun and are generally idle, only accreting the occasional star or gas cloud that ventures too close to the galaxy's center.

- A small fraction of them are, however, extremely active, devouring matter at a very high rate, causing the surrounding material to shine brightly across the electromagnetic spectrum, from radio waves to X-rays and gamma rays. In some cases, emission from matter in the vicinity of the black hole is so intense that the core of the galaxy outshines the stars. These objects appear as point sources in the sky, like stars, and are known as quasars – short for quasi-stellar sources.

- Quasars allow scientists to study gravity in the very strong field of the supermassive black holes. In addition, comparing the properties of quasars with those of other galaxies that host either active or passive black holes can reveal interesting aspects about the evolution of galaxies over cosmic history.

- But one other aspect piqued the interest of two scientists from the Arcetri Astrophysical Observatory in Firenze, Italy: they realized that quasars can be used as probes of the expansion history of the Universe. 59)

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Figure 46: This artistic view shows an accreting supermassive black hole at the core of a galaxy (image credit: ESA, C. Carreau)

Legend to Figure 46: Actively accreting supermassive black holes devour matter at very high rate, causing the surrounding material to shine brightly across the electromagnetic spectrum, from radio waves to X-rays and gamma rays. In some cases, emission from matter in the vicinity of the black hole is so intense that the core of the galaxy outshines the total luminosity of its stars: as a result, these objects appear as point sources in the sky, like stars, and are known as quasars – short for quasi-stellar sources.

As the accreted material flows towards the black hole through a disc, it is heated by friction and shines brightly at visible and ultraviolet wavelengths, shown here in red and yellow, respectively. Part of the light emitted by the disc interacts with highly energetic electrons in a corona near the disc (shown in blue), receiving an extra energy boost and turning into X-rays.

• August 20, 2015: This new image of powerful remnants of dead stars and their mighty action on the surrounding gas from ESA's XMM-Newton X-ray observatory reveals some of the most intense processes taking place at the center of our galaxy, the Milky Way (Figure 47). The bright, point-like sources that stand out across the image trace binary stellar systems in which one of the stars has reached the end of its life, evolving into a compact and dense object – a neutron star or black hole. Because of their high densities, these compact remnants devour mass from their companion star, heating the material up and causing it to shine brightly in X-rays. 60) 61)

- The central region of our galaxy also contains young stars and stellar clusters, and some of these are visible as white or red sources sprinkled throughout the image, which spans about one thousand light-years. Most of the action is occurring at the center, where diffuse clouds of gas are being carved by powerful winds blown by young stars, as well as by supernovas, the explosive demise of massive stars.

- The supermassive black hole sitting at the center of the Galaxy is also responsible for some of this action. Known as Sagittarius A*, this black hole has a mass a few million times that of our Sun, and it is located within the bright, fuzzy source to the right of the image center.

- While black holes themselves do not emit light, their immense gravitational pull draws in the surrounding matter that, in the process, emits light at many wavelengths, most notably X-rays. In addition, two lobes of hot gas can be seen extending above and below the black hole.

- Astronomers believe that these lobes are caused either directly by the black hole, which swallows part of the material that flows onto it but spews out most of it, or by the cumulative effect of the numerous stellar winds and supernova explosions that occur in such a dense environment.

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Figure 47: The central regions of our galaxy, the Milky Way, seen in X-rays by ESA's XMM-Newton X-ray observatory (image credit: ESA/XMM-Newton, G. Ponti et al., 2015)

Legend to Figure 47: This image portrays powerful remnants of dead stars and their mighty action on the surrounding gas, showing us an unprecedented view of the Milky Way's energetic core. It was put together in a new study by compiling all observations of this region that were performed with XMM-Newton, adding up to over one month of monitoring in total. — The image combines data collected at energies from 0.5 to 2 keV (shown in red), 2 to 4.5 keV (shown in green) and 4.5 to 12 keV (shown in blue). It spans about 2.5º across, equivalent to about one thousand light-years. — This image, showing us an unprecedented view of the Milky Way's energetic core, was put together in a new study by compiling all observations of this region that were performed with XMM-Newton, adding up to about one and a half months of monitoring in total.

• February 19, 2015: Astronomers have discovered that the winds from supermassive black holes at the center of galaxies are blasted out in all directions. This new finding was made possible by observations with ESA's XMM-Newton and NASA's NuSTAR X-ray telescopes and it supports the picture of black holes having a significant impact on star formation of their host galaxy. 62) 63)

- At the core of every massive galaxy in the Universe sits a supermassive black hole, with a mass some millions or billions of times that of our Sun. Some of these black holes are active, meaning that there is a frenzied flow of matter in their vicinity: the intense gravitational pull of the black hole causes matter to spiral towards it in an accretion disc and at the same time part of that matter is cast away through powerful winds.

- For the past few decades, astronomers have investigated if and how these inflows and outflows of matter may have large-scale effects on the host galaxy and, in particular, on its star formation activity. While such an interaction would explain several observations, including the correlation between the mass of stars in the bulge of a galaxy and the mass of its central black hole, it is by no means obvious that the black hole could have an impact on its host galaxy as a whole.

- "Black holes are powerful objects, but their gravitational influence does not extend much beyond the very inner parts of a galaxy," explains Emanuele Nardini of Keele University, UK. "If black holes are really to influence the star-forming activity of an entire galaxy, there must be a feedback mechanism connecting the two on a global scale." One possibility is that the winds driven by the black hole's accretion activity play a role and, as reported in the journal Science, Nardini and collaborators have obtained the first solid evidence supporting this scenario.

- The team have looked at PDS 456, a galaxy that lies just over two billion light-years away and that hosts an exceptionally active black hole with a mass of one billion Suns. PDS 456 is a quasar, a class of galaxies that appear as a point source because the brightness produced by their stars is outshone by the emission from the activity of the central black hole.

- Due to the high temperatures caused by friction in the material flowing towards the black hole, the accreted material shines brightly at optical and ultraviolet wavelengths, and subsequent interactions of this light with highly energetic electrons in the immediate surroundings of the disc also produce copious amounts of X-rays. A comprehensive view of the accreting activity of this quasar could be obtained by observing PDS 456 simultaneously with ESA's XMM-Newton X-ray observatory and NASA's NuSTAR (Nuclear Spectroscopic Telescope Array ) mission. The observations were performed on four occasions in 2013, and once again in 2014.

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Figure 48: This illustration shows the powerful wind driven by the supermassive black hole at the center of a galaxy. The schematic figure in the insert depicts the innermost regions of the galaxy where a black hole accretes the surrounding matter (light grey) at a very high pace via a disc (darker grey). At the same time, part of that matter is cast away through powerful winds (image credit: NASA/JPL, Caltech)

Legend to Figure 48: A study based on joint observations with ESA's XMM-Newton and NASA's NuSTAR X-ray telescopes of the quasar PDS 456, which hosts a very active black hole, has shown that the winds driven by a black hole can be wide and almost spherical. This discovery supports the picture of black holes having a significant impact on star formation of their host galaxy.

• December 19, 2014: First impressions can be deceptive – astronomers have used ESA's X-ray satellite XMM-Newton to find a massive black hole hungrily feeding within a tiny dwarf galaxy (Figure 49), despite there being no hint of this black hole from optical observations. 64) 65)

- The galaxy, an irregular dwarf named J1329+3234, is one of the smallest galaxies yet to contain evidence of a massive black hole. Located over 200 million light-years away, the galaxy is similar in size to the Small Magellanic Cloud, one of our nearest neighboring galaxies, and contains a few hundred million stars.

- In 2013, an international team of astronomers was intrigued to discover infrared signatures of an accreting black hole within J1329+3234 when they studied it with the Wide-Field Infrared Survey Explorer (WISE).

- The same team has now investigated the galaxy further, using ESA's XMM-Newton to hunt for this black hole in X-rays – and found something very surprising.

- "The X-ray emission from J1329+3234 is over 100 times stronger than expected for this galaxy," says Nathan Secrest of George Mason University in Virginia, USA, lead author of the new study published in The Astrophysical Journal. "We would typically expect to find low-level X-ray emission from stellar-mass black holes within the galaxy, but what we found instead was emission consistent with a very massive black hole."

- The combined X-ray and infrared properties of this galaxy can only be explained by the presence of a massive black hole residing in J1329+3234, similar to the supermassive black holes found at the centers of much more massive galaxies.

- While the exact mass of the black hole is not known, it must be at least 3000 times as massive as the Sun, although it is likely to be much more massive than that. If the black hole in J1329+3234 is similar to known low-mass supermassive black holes, then it has a mass of around 150, 000 times that of the Sun.

- A feeding black hole at the center of a galaxy is known as an AGN (Active Galactic Nucleus). In the region surrounding the black hole, material from the galaxy emits intensely bright radiation as it swirls inwards towards the center of the galaxy and is devoured by the black hole. AGNs powered by massive black holes are commonplace in large galaxies, but they appear to be rarer in galaxies without a central "bulge" of stars – dwarf galaxies being a key example.

- "This is a really important discovery," says co-author Shobita Satyapal, also from George Mason University. "It's interesting enough that such a tiny galaxy has such a large black hole, but this also raises questions about how these black holes form in the first place."

- Astronomers believe that the "seeds" of massive black holes formed very early on in the Universe, along with the first generation of stars. These seed black holes then grew into massive black holes via a string of galaxy mergers. As the galaxies merged, so did their central black holes.

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Figure 49: This image depicts the X-ray emission from dwarf galaxy J1329+3234 (center in this image), and from a background AGN (lower right), measured by XMM-Newton in June 2013 [image credit: SA/XMM-Newton, N. Secrest, et al. (2015)]

Legend to Figure 49: The image is constructed from 2-10 keV X-ray emission and has been smoothed. The color code represents the intensity of X-ray emission with blue being more intense and red less intense. The white bar indicates a width of 10 arcseconds, equivalent to 3.3 kpc at the distance of this galaxy. North is up, east to the left.

• December 15, 2015: Today marks the release of the first papers to result from the XXL survey, the largest survey of galaxy clusters ever undertaken with ESA's XMM-Newton X-ray observatory. The gargantuan clusters of galaxies surveyed are key features of the large-scale structure of the Universe and to better understand them is to better understand this structure and the circumstances that led to its evolution. The first results from the survey, published in a special issue of Astronomy and Astrophysics, hint at the answers and surprises that are captured in this unique bank of data and reveal the true potential of the survey. 66)

- The XXL project, the largest XMM-Newton observing program to date, set itself the ambitious task of mapping galaxy clusters back to a time when the Universe was just half of its present age. Its aim was to trace the evolution of the large-scale structure of the Universe.

• November 20, 2014: Mission extension for XMM-Newtom. During its meeting at ESAC (European Space Astronomy Center), near Madrid, on 19 November, the SPC (Science Program Committee) gave the green light for the flotilla of spacecraft to continue their key scientific endeavors for at least another two years. 67)

- After a comprehensive review by the Science Program's advisory structure of the current operational status and likely scientific return of each mission in the future, the SPC agreed to continue funding for six ESA-led missions (Cluster, INTEGRAL, Mars Express, PROBA-2, SOHO and XMM-Newton) for the period 1 January 2015 – 31 December 2016.

- XMM-Newton, the Hubble Space Telescope and INTEGRAL will continue to provide complementary, multi-wavelength observations of the Universe. These will span studies of the Solar System, planet-hosting stars, exploding stars, black holes, and the evolution of galaxies.

• July 11, 2014: ESA's XMM-Newton observatory has helped to uncover how the Universe's first stars ended their lives in giant explosions. Astronomers studied the gamma-ray burst GRB130925A – a flash of very energetic radiation streaming from a star in a distant galaxy 5.6 billion light years from Earth – using space- and ground-based observatories. They found the culprit producing the burst to be a massive star, known as a blue supergiant. These huge stars are quite rare in the relatively nearby Universe where GRB130925A is located, but are thought to have been very common in the early Universe, with almost all of the very first stars having evolved into them over the course of their short lives. 68) 69)

- But unlike other blue supergiants we see nearby, GRB130925A's progenitor star contained very little in the way of elements heavier than hydrogen and helium. The same was true for the first stars to form in the Universe, making GRB130925A a remarkable analogue for similar explosions that occurred just a few hundred million years after the Big Bang.

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Figure 50: This artist's impression depicts a region of an exploding blue supergiant. These stars are quite rare in the relatively nearby Universe, but are thought to have been very common in the early Universe, with almost all of the first stars having evolved into them over the course of their short lives. The illustration shows a hot cocoon of gas (shown in red) surrounding a relativistic jet emerging from the blue supergiant (image credit: NASA/Swift/A. Simonnet, Sonoma State University)

Legend to Figure 50: Astronomers used a number of space- and ground-based observatories, including ESA's XMM-Newton to study the gamma-ray burst GRB130925A – a flash of very energetic radiation streaming from a blue supergiant in a galaxy 5.6 billion light years from Earth. They found evidence that this star contained very little in the way of elements heavier than hydrogen and helium. The same was true for the first stars to form in the Universe, making GRB130925A a remarkable analog for similar explosions that occurred just a few hundred million years after the Big Bang.

General status of the XMM-Newton spacecraft as of May 2014:70)

All AOCS units are currently operating on the prime branch and do not show any significant sign of degradation. The complete AOCS redundant units are available and untouched for nominal operations. Relevant ON/OFF cycles and life limited parts of Inertia Measurement Units (IMU), Reaction Wheels (RW), Flow Control Valves (FCV), Catalyst Bed Heaters (CBH), and Latch Valves (LV) are well below critical margins. Performances of all the subunits (e.g. thrusters and reaction wheels) do not show any unexpected major degradation, where two of the reaction wheels show indication of increased bearing noise which however has been reduced by various counter measures . Thrusters degradation at current degradation rate is estimated to be 10% during the extended lifetime by manufacturer. The A unit of the star tracker suffers from some blemish pixels. Right now operations that would require the use of those pixels are avoided with a ground based filtering by the Flight Dynamics system. However a new approach using the on board offset table for the star tracker to correct for the wrong behavior of hot pixels is under investigation for implementation if the number of hot pixels would get to high.

- Automation on board: The XMM-Newton platform design is to fail safe after any single anomaly and remain at a safe attitude until recovery is possible from ground command. In case of loss of contact, the mission operations concept is designed to ensure both platform and instruments remain safe for at least 36 hours. The CDMU "autonomy" functions are limited to a time-tagged command buffer and a number of mission specific monitoring and control tasks. The timetagged command buffer is used to: ensure instruments are commanded safe before perigee passage; maintain instrument thermal control during eclipse; and since a platform thermistor failure in 2009 to maintain tank heater control. A monitor and control task to command instruments safe in case of ESAM was provided in the launch version of the CDMU software. A new task was uplinked in 2001 to monitor and control the prime instrument CCD temperature. In 2014 it is planned to uplink and commission two further tasks: a thermal duty cycle (TDC) function and a thermal closed loop (TCL) function. The former allows parallel control of a number of heater circuits by commanding the heater transistor switch to a predefined duty cycle. This will be used primarily for fine control of heater power input needed for each tank replenishing event. The latter allows closed loop control of a number of heater circuits by monitoring the associated thermistor temperature and commanding the heater switch to maintain a limit cycle between minimum and maximum values. This will be used both for tank temperature control in between tank replenishing events and for autonomous instrument heater control during and after eclipses affected by long ground station gaps.

- Automation on ground: The original design of XMM-Newton operations was based on a continuous ground station coverage and cannot be changed anymore, since the spacecraft has no mass memory on board. Telecommanding and Telemetry reception is therefore handled "live" from the MCS (Mission Control System) with individual commands, which are however for routine operations prepared and bundled into an automated timeline. The timeline runs in real time and sends all the required commands. Verification of telecommanding is done via the SCOS 2000 system through the SPACON (SPAcecraft CONtroller) supported by out-off-limits warnings from SCOS 2000. Additional commands or recovery actions have to be send manually by the SPACON using flight operation procedures. In order to support the SPACON in his activities a system is currently been developed to automate some of these procedures using a platform called MOIS (Manufacturing and Operations Information System).

Offline, using the so called MOIS Scheduler, a plan is generated for each revolution and contains the orbit constraints, the ground station schedule and the science activities. The automated version of a traditional flight operation procedure is then linked to a generic event of the timeline and executes at runtime. Furthermore the MOIS Executor is able to read telemetry and react to it by sending telecommand and interacting with the mission control system. Repeating operations are hence triggered automatically and will reduce human intervention. Automation is especially useful to execute routine operations which are not part of the timeline such as eclipse operations or calibration activities. Moreover, MOIS has been set up to cope with telemetry out-of-limit. In case of anomaly, the mission control system raises a warning flag and notifies the automation system. The MOIS Event Monitor initiates then the execution of the recovery procedure. The operational philosophy for introducing the automation on ground is to start with nominal operations and eventually after gaining experience adding as well contingency operations.

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Figure 51: Power generated from the solar arrays. The sinusoidal variation is a seasonal effect where more power is generated in winter since the spacecraft is closer to the sun. The red line shows the peak power consumption. The power margin in 2014 is of the order of 600 W (image credit: ESA)

Fuel

remaining
use per year

55 kg
3.2 kg

Solar array power

Maximum required
Current margin

1350 W
600 W

Gyros

Usage

< 21 %

Reaction wheels

Usage

< 40 %

Rf switches


Tansponder switches

Usage

Stuck at one position. Back up not used instead transponders are switched
TXA LCL switches < 900
TXB LCL switches < 900
(Qualified to 50000)

Table 5: Usage of important components of XMM-Newton

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Figure 52: Planned automation flow of the XMM-Newton Mission Control system (image credit: ESA)

- In summary, ESA's XMM-Newton X-ray space observatory is after nearly 15 years in orbit operating without any significant unexpected degradation. The only life limiting item being the hydrazine reserves on board has been extended by various measures from 2019 to 2027 with a potential improvement to 2030 currently under investigation. The main factor for fuel saving was a change to the on-board software of the Attitude Control Computer to allow operating all four reaction wheels in parallel instead of only running three of them as done previously. This method offered as well the possibility to apply measures against increased bearing noise, which has been detected on two of the XMM-Newton reaction wheels. We have been able to reduce the bearing noise fraction from ~40 % to less than 8 % for RW1 and below the detection threshold for RW2. Given this highly encouraging perspective the mission is being prepared for a potential future of up to 15 years, developing plans to operate the Hydrazine Propulsion System in the so-called near fuel depletion regime at the end of the technical life limit. The ground segment hardware has been migrated to state of the art generic virtualized systems with higher performance and greater redundancy. Furthermore ground and on board automation possibilities are being developed to make operations robust and more efficient.

• April 22, 2014: A pair of supermassive black holes in orbit around one another have been spotted by XMM-Newton. This is the first time such a pair have been seen in an ordinary galaxy. They were discovered because they ripped apart a star when the space observatory happened to be looking in their direction. 71) 72)

- Most massive galaxies in the Universe are thought to harbor at least one supermassive black hole at their center. Two supermassive black holes are the smoking gun that the galaxy has merged with another. Thus, finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes.

- To date, only a few candidates for close binary supermassive black holes have been found. All are in active galaxies where they are constantly ripping gas clouds apart, in the prelude to crushing them out of existence.

- In the process of destruction, the gas is heated so much that it shines at many wavelengths, including X-rays. This gives the galaxy an unusually bright center, and leads to it being called active. The new discovery, reported by Fukun Liu, Peking University, Beijing, China, and colleagues, is important because it is the first to be found in a galaxy that is not active.

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Figure 53: Artist's impression of a binary supermassive black hole system (image credit: ESA, C. Carreau)

• March 5, 2014: Astronomers have used NASA's Chandra X-ray Observatory and the ESA's XMM-Newton to show a supermassive black hole six billion light years from Earth is spinning extremely rapidly. This first direct measurement of the spin of such a distant black hole is an important advance for understanding how black holes grow over time. 73)

- Multiple images of a distant quasar are visible in this combined view from NASA's Chandra X-ray Observatory and the Hubble Space Telescope. The Chandra data, along with data from ESA's XMM-Newton, were used to directly measure the spin of the supermassive black hole powering this quasar. This is the most distant black hole where such a measurement has been made.

- Reis and his colleagues determined the spin of the supermassive black hole that is pulling in surrounding gas, producing an extremely luminous quasar known as RX J1131-1231 (RX J1131 for short). Because of fortuitous alignment, the distortion of space-time by the gravitational field of a giant elliptical galaxy along the line of sight to the quasar acts as a gravitational lens that magnifies the light from the quasar. Gravitational lensing, first predicted by Einstein, offers a rare opportunity to study the innermost region in distant quasars by acting as a natural telescope and magnifying the light from these sources.

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Figure 54: RX J1131-1231: Chandra & XMM-Newton Provide Direct Measurement of Distant Black Hole's Spin (image credit: NASA/CXC/Univ of Michigan/R. C. Reis, et al; Optical: NASA/STScI)

• November 13, 2013: Astronomers studying a black hole in our Galaxy with ESA's XMM-Newton observatory have made a surprising discovery about the cocktail of particles that are ejected from its surroundings. Stellar-mass black holes are often found feasting on material from a companion star. Matter flows from the star towards the black hole, circling in a disc around it with a temperature so high that it emits X-rays. 74) 75)

- The black hole can be a fussy eater: instead of swallowing all of the material, it sometimes pushes a fraction of it away in the form of two powerful jets of particles. Because these jets release mass and energy into the surroundings, the black hole has less material to feed on. By studying the composition of the jets, we can learn more about the feeding habits of black holes.

- Observations at radio wavelengths have already found that black hole jets contain electrons moving at close to the speed of light. But, until now, it was not clear whether the negative charge of the electrons is complemented by their anti-particles, positrons, or rather by heavier positively-charged particles in the jets, like protons or atomic nuclei.

- In a new study, astronomers have used XMM-Newton to study a black hole binary system called 4U1630–47, well known to show outbursts of X-rays over periods of months and years.

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Figure 55: Artist's impression of a black hole feasting on matter from its companion star in a binary system. Matter flows from the star towards the black hole and forms an accretion disc with a temperature so high that it emits X-rays. The black hole can be a fussy eater: instead of swallowing all of the material, it sometimes pushes a fraction of it away in the form of two powerful jets of particles (image credit: ESA/ATG medialab)

Legend to Figure 55: A team of astronomers studying the jets of the binary system 4U1630-47 have confirmed that black hole jets not only consist of electrons but also contain heavier particles, like protons or atomic nuclei. This means that jets can carry mass and energy away from the black hole in much larger amounts than previously thought.

• In July 2013, ESA released a new catalog from the XMM-Newton space telescope which provides an unprecedented cosmic X-ray library for the exploration of the extreme Universe. The third XMM-Newton Serendipitous Source Catalog (3XMM-DR4) contains more than half a million sources, all of which are provided to a better quality than ever before. Improved data processing means that source identification is more reliable, and fainter objects are detected. 76)

- The catalogue provides an exceptional dataset for generating large, well-defined samples of objects such as active galaxies (which dominate the detections in this catalogue), clusters of galaxies, interacting compact binaries, and active stellar coronae. This vast inventory is also home to some of the rarest and most extreme phenomena in the Universe, such as tidal disruption events - when a black hole swallows another star, producing prodigious outbursts of X-ray emission.

- "The catalogue provides plenty of scope for new discoveries as well as in-depth studies of large samples," says Professor Mike Watson of the University of Leicester, who leads the XMM-Newton Survey Science Center (SSC). "XMM-Newton is pre-eminent amongst current X-ray missions in its ability to perform 'survey' science, with a chance to find previously undetected objects and then explore their properties."

- The sources in the 3XMM catalogue are identified and isolated from serendipitous data recorded by XMM-Newton's EPIC X-ray cameras. In each of the 600-700 observations made each year, around 70 extra sources are captured in addition to the target object which usually only takes up a small fraction of the field of view. Covering observations between February 2000 and December 2012, the catalogue contains some 531 261 X-ray source detections relating to 372 728 unique X-ray sources.

- "The third XMM-Newton Serendipitous Source Catalogue shows how much added value can be gained from the observations," notes Watson. "I'd like to pay tribute to the efforts of the whole team which were crucial to completing this major undertaking."

- Natalie Webb of the Institut de Recherche en Astrophysique et Planétologie, Toulouse is taking over from Leicester's Simon Rosen as the SSC Manager for the next phase of the project. She comments: "Previous versions of the catalogue have yielded unexpected and exciting results, and with around 50 per cent more data now available, there should be plenty more to come. I look forward to the SSC continuing to play its leading role in this area."

- Coinciding with the publication of the 3XMM catalogue, a new version of the XMM-Newton Science Archive (XSA) is released, providing convenient access to the catalogue. The most visible change in XSA is the new web-based interface, but behind the scenes many changes have been introduced to make it easier and quicker to find and use the data.

- The 3XMM-DR4 catalogue is the sixth publicly released XMM-Newton X-ray source catalogue produced by the XMM-Newton Survey Science Center consortium on behalf of ESA.

- The XMM-Newton Survey Science Center, led by Professor Mike Watson at the University of Leicester, is a consortium of the following institutions:

University of Leicester, United Kingdom

Mullard Space Science Laboratory, University College London, United Kingdom

Institute of Astronomy, Cambridge, United Kingdom

Max-Planck Institut für extraterrestrische Physik, Garching, Germany

Astrophysikalisches Institut, Potsdam, Germany

Service d'Astrophysique, CEA/DSM/Dapnia, Saclay, France

Institut de Recherche en Astrophysique et Planétologie, Toulouse, France

Observatoire Astronomique de Strasbourg, France

Instituto de Fisica de Cantabria, Santander, Spain

Osservatorio Astronomico di Brera, Milan, Italy.

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Figure 56: All-sky map of the third XMM-Newton Serendipitous Source Catalog, 3XMM-DR4 (image credit: ESA/XMM-Newton/EPIC/Mike Watson)

Legend to Figure 56: The sources in the 3XMM catalog are identified and isolated from serendipitous data recorded by XMM-Newton's EPIC X-ray cameras. In each of the 600-700 observations made each year, around 70 extra sources are captured in addition to the target object which usually only takes up a small fraction of the field of view. Covering observations between February 2000 and December 2012, the catalog contains some 531 261 X-ray source detections relating to 372 728 unique X-ray sources. — The inset shows an expanded view of the small region indicated where the individual sources can be seen.

• June 4, 2013: The XMM-Newton mission is in operation; it is detecting more X-ray sources than any previous satellite and is helping to solve many cosmic mysteries of the violent Universe, from what happens in and around black holes to the formation of galaxies in the early Universe. It is designed and built to return data for at least a decade. It is the biggest science satellite ever built in Europe. Its telescope mirrors are the most sensitive ever developed in the world, and with its sensitive detectors, it sees much more than any previous X-ray satellite. 77)

- XMM-Newton has been able to measure for the first time the influence of the gravitational field of a neutron star on the light it emits. This measurement provides much better insight into these objects. Neutron stars are among the densest objects in the Universe — a sugarcube-sized piece of a neutron star would weigh over a thousand million tonnes. Neutron stars are the remnants of heavy stars that end their life in a supernova explosion. In such cataclysmic events, most of the stellar matter is ejected into space (to eventually become the building blocks of all matter in the Universe, including ourselves). Part of what remains then collapses under its own gravity.

- Scientists believe that, in a neutron star, the density and the temperatures are similar to those existing a fraction of a second after the Big Bang when the primordial soup of matter in the Universe was ‘broken' into its most fundamental constituents. They assume that when matter is tightly packed as it is in a neutron star, it goes through important changes. Protons, electrons, and neutrons — the components of atoms — fuse together. It is possible that even the building blocks of protons and neutrons, the so-called quarks, get crushed together.

- Scientists have spent the last decades trying to identify the nature of matter in neutron stars. To do this, they need to know some important parameters, very precisely. If you know a star's mass and radius, or the relationship between them, you can obtain its density. However, no instrument was advanced enough to perform the measurements needed, until now. Thanks to ESA's XMM-Newton observatory, astronomers have been able to obtain the mass-to-radius ratio of a neutron star for the first time and acquire the first clues about its composition. These clues suggest that neutron stars contain normal, non-exotic matter, although they are not conclusive. Scientists say this is a ‘key first step' and that they will keep on with the search.

- This measurement was a first in astronomical observation and it is considered a huge achievement. The method consists of determining the compactness of the neutron star in an indirect way. The gravitational pull of a neutron star is immense — thousands of million times stronger than the Earth's. This makes the light emitted by the neutron star lose energy. This energy loss is called a gravitational ‘redshift'. The measurement of this redshift by XMM-Newton indicated the strength of the gravitational pull, and revealed the star's compactness.

• February 27, 2013: A rapidly rotating supermassive black hole has been found in the heart of a spiral galaxy by ESA's XMM-Newton and NASA's NuSTAR space observatories, opening a new window into how galaxies grow. Supermassive black holes are thought to lurk in the center of almost all large galaxies, and scientists believe that the evolution of a galaxy is inextricably linked with the evolution of its black hole. 78) 79)

- How fast a black hole spins is thought to reflect the history of its formation. In this picture, a black hole that grows steadily, fed by a uniform flow of matter spiralling in, should end up spinning rapidly. Rapid rotation could also be the result of two smaller black holes merging.

XMM-Newton_Auto12

Figure 57: ESA's XMM-Newton and NASA's NuSTAR have detected a rapidly rotating supermassive black hole in the heart of spiral galaxy NGC 1365. The rate at which a black hole spins encodes the history of its formation. An extremely rapid rotation could result from either a steady and uniform flow of matter spiralling in via an accretion disc (as shown in this artist impression) or as a result of the merger of two galaxies and their smaller black holes (image credit: NASA/JPL, Caltech) 80)

Legend to Figure 57: Depicted in this image is an outflowing jet of energetic particles, believed to be powered by the black hole's spin. The regions near black holes contain compact sources of high energy X-ray radiation thought, in some scenarios, to originate from the base of these jets. The nature of the X-ray emission enables astronomers to see how fast matter is swirling in the inner region of the disc, and ultimately to measure the black hole's spin rate.

• January 24, 2013: New observations of a highly variable pulsar using ESA's XMM-Newton are perplexing astronomers. Monitoring this pulsar simultaneously in X-rays and radio waves, astronomers have revealed that this source, whose radio emission is known to 'switch on and off' periodically, exhibits the same behavior, but in reverse, when observed at X-ray wavelengths. It is the first time that a switching X-ray emission has been detected from a pulsar, and the properties of this emission are unexpectedly puzzling. As no current model is able to explain this switching behavior, which occurs within only a few seconds, these observations have reopened the debate about the physical mechanisms powering the emission from pulsars. 81) 82)

- Few classes of astronomical objects are as baffling as pulsars – which were discovered as flickering sources of radio waves and soon after interpreted as rapidly rotating and strongly magnetized neutron stars. Even though about 2000 pulsars have been found since the first was discovered in 1967, a detailed understanding of the mechanisms that power them still eludes astronomers.

- "There is a general agreement about the origin of the radio emission from pulsars: it is caused by highly energetic electrons, positrons and ions moving along the field lines of the pulsar's magnetic field, and we see it pulsate because the rotation and magnetic axes are misaligned," explains Wim Hermsen from SRON, the Netherlands Institute for Space Research in Utrecht, The Netherlands. "How exactly the particles are stripped off the neutron star's surface and accelerated to such high energy, however, is still largely unclear," he adds.

- "Many pulsars have a rather erratic behavior: in the space of a few seconds, their emission becomes weaker or even disappears for a while, just to go back to the previous level after some hours," says Hermsen. "We do not know what causes such a switch, but the fact that the pulsar keeps memory of its previous state and goes back to it suggests that it must be something fundamental."

- Recent studies indicate that the switch between what are usually referred to as 'radio-bright' and 'radio-quiet' states is correlated to the pulsar's dynamics. As pulsars rotate, their spinning period slows down gradually, and in some cases the slow-down process has been observed to accelerate and slow down again, in conjunction with the pulsar switching between radio-bright and quiet states. The existence of correlated variations in both the rotation and emission suggest a connection between a pulsar's immediate vicinity and, on a grander scale, its co-rotating magnetosphere, which may extend up to about 50 000 km for objects like PSR B0943+10. In order for the radio emission to vary so radically on the short timescales observed, the pulsar's global environment must undergo a very rapid – and reversible – transformation.

XMM-Newton_Auto11

Figure 58: Artist's impression of a pulsar in radio-bright mode. This illustration shows a pulsar with glowing cones of radiation stemming from its magnetic poles – a state referred to as 'radio-bright' mode (image credit: ESA/ATG medialab)

• December 11, 2012: At about 06:51 hrs UT on 11 December 2012, during XMM-Newton revolution 2382, an event was registered in the focal plane of the EPIC MOS1 instrument. The characteristics of the event were reminiscent of a similar event registered in the MOS1 focal plane on 9 March 2005 and of other less energetic events registered in the MOS1 focal plane on 17 September 2001, the MOS2 focal plane on 12 August 2002, and the pn focal plane on 19 October 2000, which were attributed to micrometeoroid impacts scattering debris into the focal plane. In each MOS case a bright flash of light caused data buffer overflows for the CCDs across the whole focal plane, and in all cases a number of new hot or defective pixels were subsequently mapped and masked. The consequences of the event on 9 March 2005 were more significant meaning, in addition, the loss of one of the seven MOS1 CCD detectors, namely CCD6: 83)

- After the event on 11 December 2012, another peripheral CCD, namely MOS1 CCD3, is significantly damaged.

- No impact on the other X-ray instruments has been observed.

- Scientific observations are continuing normally with XMM-Newton, including MOS1, but now without data from CCD3, or from CCD6 as a result of the 2005 event.

- The science impact of the loss of MOS1 CCD3 is small.

CCD3 is one out of the six original peripheral CCDs of MOS1. It covers, to a first order approximation, slightly less than 1/7 (or 14%) of the geometrical area of MOS1. MOS1, in turn, only contains some 22% of the total effective area of the EPIC instrument with MOS1, MOS2 and pn, operating simultaneously. Therefore, the impact of the loss of CCD3 is zero for on-axis point sources and extended sources with radius smaller than 5.5 arcmin. For sources falling in CCD3 or for the extended emission of on-axis sources at distances larger than 5.5 arcmin, there is a 22% decrease in effective area (or 12% decrease in signal-to-noise ratio) over 14% of the field.

The combined impact of the two events from March 2005 and December 2012, which affected MOS1 CCD6 and CCD3, respectively, remains zero for on-axis point sources and a 12% reduction of the signal-to-noise in about 28% of the off-axis field of view for point-like or extended emission at radial distances larger than 5.5 arcmin.

• August 3, 2012: Astronomers have detected tell-tale luminosity fluctuations in the X-ray signal from a star that was torn apart and devoured by the supermassive black hole at the center of a distant galaxy. The fluctuations, which have a period of 200 seconds, originate from the innermost stable orbit around the black hole and represent the last signal sent by the debris of the disrupted star before disappearing beyond the black hole's event horizon. The discovery, based on data from ESA's XMM-Newton and the Japan/US Suzaku space observatories, has allowed astronomers to probe the details of matter accretion onto a supermassive black hole in the distant Universe for the first time. 84) 85)

- Black holes exist on a variety of scales, from the stellar-mass ones that derive from the collapse of massive stars to the supermassive black holes that reside at the center of most galaxies and have masses that are millions or even billions of times larger than the Sun's. Regardless of their mass, the phenomena that arise in the proximity of these extremely dense and compact objects due to their intense gravitational fields are quite similar. An effect of the accretion of the surrounding matter onto a black hole is the emission of radiation across the electromagnetic spectrum, which has been detected and studied extensively around stellar-mass and supermassive black holes alike. These observations allow astronomers to probe the behavior of gravity in its strongest regime and to test general relativity in a wide range of environments, both in our Galaxy (the Milky Way) and in more distant galaxies.

- A small fraction of the supermassive black holes hosted at the center of galaxies are undergoing 'active' accretion and feeding on a supply of gas at tremendous rates – these are the so-called AGN (Active Galactic Nuclei). However, the majority of supermassive black holes, including the one at the center of the Milky Way, are in a dormant state and only accrete matter on rare occasions, when a star happens to pass too close to it. In this case, matter on the side of the star facing the black hole experiences a stronger pull with respect to the other side, and this eventually tears the star apart. This phenomenon, referred to as tidal disruption, temporarily switches on the black hole's activity: debris from the shattered star starts orbiting around the black hole in a disc and part of it is rapidly accreted, causing a sudden boost in the luminosity of the galaxy's center, especially at the highest energies.

XMM-Newton_Auto10

Figure 59: Quasi-periodic oscillations from a disrupted star being devoured by a supermassive black hole (image credit: NASA's Goddard Space Flight Center)

Legend to Figure 59: This illustration shows an artist's impression of Sw J1644+57, a supermassive black hole hosted at the center of a distant galaxy. The black hole, previously dormant, was temporarily switched on as a star passed too close to it. Torn apart by the black hole's gravitational pull, the star was tidally disrupted and its debris started orbiting around the black hole forming a disc. Part of the debris was rapidly accreted by the black hole, causing a sudden boost in the luminosity of the host galaxy's center, especially at the highest energies.

Originally detected as a GRB (Gamma-Ray Burst) by the NASA satellite Swift, the source, named Sw J1644+57, remained exceptionally bright for a few weeks after its discovery, unlike any other known GRB. After further observations, astronomers were able to link the flaring source to a star that was being disrupted and subsequently devoured by the supermassive black hole at the center of a distant galaxy.

Further observations performed with the ESA XMM-Newton and the Japan/US Suzaku X-ray observatories revealed that Sw J1644+57 exhibits what astronomers call quasi-periodic oscillations: luminosity fluctuations that occur in a regular fashion but are only seen for a certain period of time before disappearing. Quasi-periodic oscillations are known to arise in a very special site around a black hole: the so-called innermost stable circular orbit, which depends on the black hole's mass and spin and defines its range of action. At distances larger than this limiting orbit (indicated with a yellow circle in the illustration), matter can revolve around the black hole on stable trajectories, but anything located within this orbit will inexorably precipitate towards the black hole and be quickly accreted onto it.

• June 18, 2012: In June 2012, X-ray astronomy celebrates its 50th anniversary, following the discovery of the first extra-solar cosmic X-ray source in 1962. The XMM-Newton team recognizes the rich heritage from which XMM-Newton has grown and joins astronomers all over the world in the celebration. 86)

- XMM-Newton has observed the first cosmic X-ray source different than the Sun, Scorpio X-1, which was discovered in June 1962: Figure 60 shows the high-resolution the soft X-ray spectrum of Sco X-1, taken with the RGS instrument.

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Figure 60: XMM-Newton observations of Sco X-1 (image credit; ESA

• December 20, 2011: Astronomers have discovered a very slowly rotating X-ray pulsar still embedded in the remnant of the supernova that created it. This unusual object was detected on the outskirts of the Small Magellanic Cloud, a satellite galaxy of the Milky Way, using data from a number of telescopes, including ESA's XMM-Newton. A puzzling mismatch between the fairly young age of the supernova remnant and the slow rotation of the pulsar, which would normally indicate a much older object, raises interesting questions about the origin and evolution of pulsars. 87) 88)

- The spectacular supernova explosion that marks the end of a massive star's life also has an intriguing aftermath. On the one hand, the explosion sweeps up the surrounding interstellar material creating a supernova remnant that is often characterized by a distinctive bubble-like shape, on the other hand, the explosion also leaves behind a compact object – a neutron star or a black hole. Since supernova remnants shine only for a few tens of thousands of years before dispersing into the interstellar medium, not many compact objects have been detected while still enclosed in their expanding shell.

- An international team of astronomers has now discovered one of these rarely observed pairs, consisting of a strongly magnetized, rotating neutron star – a pulsar – surrounded by the remains of the explosion that generated it.

- The newly found pulsar, named SXP 1062, is located at the outskirts of the Small Magellanic Cloud (SMC), one of the satellite galaxies of the Milky Way. SXP 1062 is an X-ray pulsar, part of a binary system in which the compact object is accreting mass from a companion star, resulting in the emission of copious amounts of X-rays. The astronomers first detected the pulsar's X-ray emission using data from ESA's XMM-Newton as well as NASA's Chandra space-based observatories. A later study of optical images of the source and its surroundings revealed the bubble-shaped signature of the supernova remnant around the binary system.

XMM-Newton_AutoE

Figure 61: This image is a composite view of the newly discovered X-ray pulsar SXP 1062 still embedded in the remnant of the supernova that created it. SXP 1062 accretes mass from its stellar companion, a massive, hot, blue 'Be' star, the two objects forming a Be/X-ray binary [image credit: ESA/XMM-Newton/L. Oskinova, University of Potsdam, Germany/M. Guerrero, Instituto de Astrofisica de Andalucia, Spain (X-ray); Cerro Tololo Inter-American Observatory/R. Gruendl & Y. H. Chu, University of Illinois at Urbana-Champaign, USA (optical)]

• June 28, 2011: With a stroke of luck, astronomers using ESA's XMM-Newton X-ray observatory have observed a neutron star in a peculiar X-ray binary system undergoing an extremely rare, intense flare. This outburst of X-rays, which lasted about four hours, was due to a sudden increase in the rate at which the neutron star was accreting matter from its companion, a blue supergiant star. By monitoring this phenomenon in unprecedented detail, the data provide the first, substantive evidence to explain such luminosity variations in this type of binary system; the flare appears to be due to the ingestion of a massive clump of matter by the neutron star. 89) 90)

- Amongst the multitude of binary star systems that shine brightly in X-rays, those consisting of a neutron star and a blue supergiant star are especially puzzling, none more so than a rather enigmatic sub-class known as Supergiant Fast X-ray Transients (SFXTs). Although usually faint - typically about 10 000 times less luminous than the brightest X-ray sources in the sky - SFXTs occasionally undergo powerful flares that boost their luminosity, for a few hours, up to the levels of the brightest X-ray binaries known. New observations of one of these curious sources have now provided evidence to explain the origin of their very peculiar behavior.

- "To investigate these huge luminosity variations we need to observe SFXTs during a flare, but this has proven to be extremely hard due to the rarity and unpredictability of such events," explains Enrico Bozzo from the ISDC Data Center for Astrophysics at the University of Geneva, Switzerland. A team led by Bozzo planned to study one particular object, the binary system IGR J18410-0535, which had been observed to flare on more than one occasion over the past few years. However, given the extremely rare occurrence of such events, which happen at most a few times per year, the main goal of the observations was to probe the system in its normal quiescent state. "Imagine then our surprise when we realized that we had caught the source while it was flaring," he adds.

- Expecting it to be rather faint, as SFXTs are most of the time, the team had secured a long observation of the source with ESA's X-ray observatory, XMM-Newton, in order to probe its properties in detail. There was no indication whatsoever that an outburst would take place during their scheduled observations. "Our discovery was a truly lucky one, and not only because XMM-Newton happened to be looking at this object during the flare," notes Bozzo. In fact, the planned observation was also, by good fortune, of the right duration. The source was observed for 12.5 hours, enabling the astronomers to record the entire extent of the flare, which lasted about 4 hours, as well as to follow the source for a few hours after it returned to its normal, dormant state. As a result of this fortuitous circumstance, Bozzo and his colleagues were able to study the event in great detail and to infer the underlying cause for the rare and powerful flares.

- It is well known that, in the case of SFXTs, the neutron star does not accrete matter through a disc, because the wind flowing out of the companion is too diffuse and fast to form one. Instead, matter is transferred directly from the wind, 'raining' onto the neutron star from virtually all directions and then being funnelled by the magnetic field lines towards its poles. The matter accretion process releases X-rays, and any abrupt outburst of this radiation, such as the detected flare, must have something to do with changes in the accretion rate.

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Figure 62: Artist's impression of a neutron star devouring a massive clump of matter (image credit: ESA/AOES Medialab)

• March 9, 2011: Astronomers working with data from several observatories, including ESA's XMM-Newton, have discovered the most distant, mature galaxy cluster yet. The cluster is seen as it was when the Universe was only about a quarter of its current age. In contrast to other structures observed in the young Universe, this object is already in its prime, as is evident from its diffuse X-ray emission and evolved population of galaxies. This shows that fully-grown galaxy clusters were already in place this early in cosmic history. 91) 92)

- According to the most widely accepted theory, structure in the Universe grows in a hierarchical fashion: under the effect of gravity, smaller objects build up first, and then accrete into progressively larger ones. This is why galaxy clusters, the largest cosmic structures that are bound by gravity, are the last to form. When these enormous assemblies of galaxies, hot gas and dark matter appeared in the cosmos is still an open question, and the urge to resolve this question drives astronomers to observe galaxy clusters that are as distant as possible.

- Given that galaxy clusters are scarce, and even more so at high redshifts where they are still taking shape, these observations are challenging. "This is why discovering a galaxy cluster at a redshift of 2 feels like unearthing a rare and valuable gem," says Raphael Gobat from the Commissariat à l'Énergie Atomique (CEA), in France, who led the extensive study that revealed what appears to be the most distant galaxy cluster detected yet.

- CL J1449+0856, the newly-found galaxy cluster, is seen at an era when the Universe was only 3 billion years old — less than a quarter of its present age. "As well as being at a record-breaking distance, what makes this object rather unique is that it's not a proto-cluster undergoing formation, as are many that have been detected at high redshifts, but it is already mature, a proper galaxy cluster," adds Gobat.

- A signature of its advanced evolutionary state is the diffuse X-ray emission associated with the cluster, observed with ESA's XMM-Newton. "Only galaxy clusters that have had time to fully develop, collapsing under the influence of their own gravity, are visible in X-rays," explains Alexis Finoguenov from the Max-Planck-Institut für extraterrestrische Physik (MPE), in Germany, co-author of the paper in which the result is presented. The X-ray emission originates from the hot intra-cluster gas: subject to the cluster's gravitational potential, the gas is compressed and heated to temperatures of over 10 million Kelvin, and shines at X-ray wavelengths.

• In December 2010, the XXL survey, an XMM-Newton Very Large Program, has been granted time to map two extragalactic regions of 25 deg2, at a depth of ~5 x10-15 erg/cm2/s (using 10 ks observations). While the main goal of the project is to constrain the Dark Energy equation of state using clusters of galaxies 93) , it will also have lasting legacy value for cluster scaling laws and studies of AGNs and the diffuse XRB (X-Ray Baxkground). 94)

• October 14, 2010: Astronomers using XMM-Newton and other world-class X-ray telescopes have probed a curious source, which emits flares and bursts just like a magnetar but lacks the extremely high external magnetic field typical of these objects. The detection of this source, which could be powered by a strong, internal magnetic field hidden to observations, may mean that many 'ordinary' pulsars are dormant magnetars waiting to erupt. 95) 96)

- Massive stars remain objects of curiosity even well after their demise. Ending their lives in dramatic fashion, as supernova explosions, they leave a very dense and compact remnant behind - a neutron star or a black hole, depending on the mass of the star. These remnants, characterized by intense gravitational fields, are the source of some extremely energetic events and give rise to a variety of interesting phenomena which can be observed throughout the entire electromagnetic spectrum.

- Neutron stars, in particular, derive from the collapse of stars originally as massive as 8 to 25 times the mass of the Sun and they harbor magnetic fields a million times stronger than the strongest ones ever produced on Earth. Spinning neutron stars can be observed as pulsating sources -hence the name, pulsars- with exceptionally short periods, ranging from about one thousandth of a second to ten seconds. Powerful beams of electromagnetic radiation are created by jets of energetic particles that stream out above the magnetic poles of the star; the 'blinking' effect arises because the pulsar's magnetic dipole is not always aligned with its axis of rotation. Young pulsars rotate extremely fast but release rotational energy and slow down as they age: older pulsars have thus longer periods than younger ones. By measuring the rate at which a pulsar spins down it is possible to estimate the intensity of its surface dipolar magnetic field.

- Magnetars are a special class of pulsars that stand out from the crowd because of their striking characteristics: they have long rotations periods, occasionally undergo episodes of extremely enhanced emission (about 10–100 times the usual value) and produce intense, short bursts of X-rays and gamma-rays.

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Figure 63: This illustration of XMM-Newton data represents the recently discovered magnetar SGR 0418+5729 (image credit: ESA)

Legend to Figure 63: Magnetars are pulsars (spinning neutron stars) characterized by long rotations periods, occasional episodes of extremely enhanced emission (about 10–100 times the usual value) and intense, short bursts of X-rays and gamma-rays; these highly energetic events are presumed to be powered by an intense magnetic field.

• June 21, 2010: A long-sought-after emission line of oxygen, carrying the imprint of strong gravitational fields, has been discovered in the XMM-Newton spectrum of an exotic binary system composed of two stellar remnants, a neutron star and a white dwarf. Astronomers can use this line to probe extreme gravity effects in the region close to the surface of a neutron star. 97)

- Stellar remnants are the last evolutionary step of the life of stars which, after having burned their nuclear fuel, collapse into very compact and exotic objects - white dwarfs, neutron stars and black holes, depending on the mass of the stars. With an enormous mass contained in a very restricted space, these objects are extremely dense; in particular, neutron stars and black holes give rise to very strong gravitational fields and thus prove to be excellent testbeds for Einstein's theory of general relativity.

- Stars often come in pairs, and neutron stars and black holes are no exception, often being found as one component of a binary system. Due to the strong gravitational attraction that the compact remnant exerts on its companion, material from the latter flows onto the remnant forming an accretion disc. As the material in the disc spirals around the remnant, it is heated up to millions of degrees - because of internal friction - and produces copious amounts of X-rays. These systems are thus referred to as X-ray binaries.

- The object of this study, 4U 0614+091, is a very special X-ray binary, consisting of two remnants, namely a neutron star accreting mass from a white dwarf. The fact that the companion star is also a compact object is evident from the exceptionally short orbital period of the system: in fact, the two objects orbit around each other in about 50 minutes, which identifies the source as an UCXB (Ultra-Compact X-ray Binary ).

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Figure 64: This illustration of 4U 0614+091 depicts the accretion disc around the neutron star, with material flowing onto it from the white dwarf companion, and symmetrical jets of relativistic matter ejected perpendicular to both sides of the disc (image credit: ESA)

Legend to Figure 64: X-ray photons irradiating the disc are reflected by the material within it. As the gas in the disc contains a wealth of hot electrons, photons scattering off these electrons gain energy, resulting in a broadening of the spectral lines towards higher energies. Gravitational redshift and the relativistic Doppler effect broaden the spectral lines towards lower energies - a signature of the strong gravitational field of the neutron star.

• May 31, 2010: Surveying the sky, XMM-Newton has discovered two massive galaxy clusters, confirming a previous detection obtained through observations of the Sunyaev-Zel'dovich effect, the 'shadow' they cast on the Cosmic Microwave Background. The discovery, made possible thanks to a novel mosaic observing mode recently introduced on ESA's X-ray observatory, opens a new window to study the Universe's largest bound structures in a multi-wavelength approach. 98)

- Galaxy clusters are the largest gravitationally bound objects in the Universe. As such, they are extremely important probes of cosmic properties on very large scales, since they form in the densest knots of the large-scale structure, the cosmic web. Originally discovered as an excess density (or cluster) of galaxies located at the same redshift, hence the name, there is much more to these enormous structures than mere galaxies: in fact, only about one tenth of the entire mass of a galaxy cluster arises from galaxies (up to a thousand in the most massive cases), another tenth consists of hot gas, and the remainder can be attributed to dark matter.

- The gas that fills galaxy clusters is hot enough to emit X-rays — with a temperature of more than 10 million Kelvin, the gas is ionized and electrons scattering off ions are decelerated, emitting radiation in the process. From measurements of the X-ray luminosity of galaxy clusters and of the gas temperature, the total mass of these structures can be estimated. This yields clear evidence that clusters are indeed gravitationally bound structures and that their mass is dominated by the elusive and invisible dark matter.

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Figure 65: Composite X-ray and optical image of the massive galaxy cluster SPT-CL J2332-5358, with contours depicting the region of the Sunyaev-Zel'dovich effect signature (image credit: ESA/XMM-Newton; Background image: Blanco Cosmology Survey/NOAO/AURA/NSF; SZE contours: South Pole Telescope/NSF)

Legend to Figure 65: X-ray emission (in pink) along with Sunyaev-Zel'dovich effect (SZE) contours (in white) of the massive galaxy cluster SPT-CL J2332-5358, overlaid on an optical g-, r- and i-band color composite image.

• April 28, 2010: One of the teams behind ESA's XMM-Newton X-ray mission has unveiled the latest edition of their 2XMM catalog. The newest incarnation boasts an additional 42 000 entries, ratcheting up the total to over a quarter of a million X-ray sources. This unprecedented cosmic X-ray library is a valuable resource allowing astronomers to explore the extreme Universe. 99)

- This latest edition of the 2XMM catalog, an unrivalled storehouse of information on X-ray sources, is the largest ever assembled. It is constructed through the serendipitous data acquired while piggy-backing on XMM-Newton's normal observing program, which is based on competitive bids from the astronomical community. XMM-Newton makes over 600 observations each year but the target object typically only takes up a tiny fraction of the field of view of 30 arcmin, equivalent to the diameter of the full Moon. Keen not to waste such an opportunity, the images are also searched for additional X-ray sources, with the findings being accumulated in the 2XMM catalogue for future reference. On average, an extra seventy sources additional to the main object of interest are found.

• January 20, 2010: Observations of faint and distant galaxy groups made with the European Space Agency's XMM-Newton observatory have been used to probe the evolution of dark matter. The results of the study are reported in the 20 January issue of The Astrophysical Journal. 100)

- Dark matter is a mysterious, invisible constituent of the Universe which only reveals itself through its gravitational influence. Understanding its nature is one of the key open questions in modern cosmology. In one of the approaches used to address this question astronomers use the relationship between mass and luminosity that has been found for clusters of galaxies which links their X-ray emissions, an indication of the mass of the ordinary (baryonic) matter alone, and their total masses (baryonic plus dark matter) as determined by gravitational lensing.

- To date the relationship could only be established for nearby clusters. New work by an international collaboration, including the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratory of Astrophysics of Marseilles (LAM), and Lawrence Berkeley National Laboratory (Berkeley Lab), has made major progress in extending the relationship to more distant and smaller structures than was previously possible.

- To establish the link between X-ray emission and underlying dark matter, the team used one of the largest samples of X-ray-selected groups and clusters of galaxies, produced by the ESA's X-ray observatory, XMM-Newton.

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Figure 66: This image shows the galaxy density in the COSMOS field, with colors representing the redshift of the galaxies (ranging from redshift of 0.2 (depicted in blue) to 1 (depicted in red). The X-ray contours (in pink) show the extended X-ray emission as observed by XMM-Newton (image credit: ESA)

Legend to Figure 66: The Cosmic Evolution Survey (COSMOS) is an astronomical survey designed to probe the formation and evolution of galaxies as a function of cosmic time (redshift) and large scale structure environment. The survey covers a 2 square degree equatorial field with imaging by most of the major space-based telescopes, including Hubble and XMM-Newton, and a number of ground-based telescopes.

• On December 10, 2009 the XMM-Newton spacecraft was 10 year on orbit. Over the course of the past 10 years XMM-Newton, in tandem with NASA's Chandra X-ray Observatory, has revolutionized the field of X-astronomy. The complementary capabilities of the two X-ray observatories, with superb spectroscopy from XMM-Newton and exquisite imaging from Chandra, have provided X-ray astronomers with the scientific probes required to investigate the physical conditions and mechanisms underpinning the most energetic and violent processes in the Universe. 101) 102)

- As the first decade of XMM-Newton draws to a close, scientists are already preparing for the future. While XMM-Newton has answered many of the questions that it was designed to address, the extraordinary view of the X-ray Universe that it has provided has also raised new questions, such as, how do black holes and matter behave under extreme conditions, can the nature of dark matter and dark energy be constrained by studying the evolution of galaxy formation, and how are chemical elements created and dispersed. The fact that the XMM-Newton spacecraft and instruments are in excellent condition and able to continue to operate for another decade means that this astronomical workhorse is well placed to pursue new challenges.

• July 1, 2009: Astronomers using ESA's XMM-Newton X-ray observatory have discovered a black hole weighing more than 500 solar masses, a missing link between lighter stellar-mass and heavier supermassive black holes, in a distant galaxy. This discovery is the best detection to date of a new class that has long been searched for: intermediate mass black holes. 103)

- Stellar-mass black holes (about three to twenty times as massive as the Sun) and supermassive black holes (several million to several thousand million times as massive as the Sun) have long been known to exist. Because of the large gap between these two extremes, scientists have speculated the existence of a third, intermediate class of black holes, with masses between a hundred and several hundred thousand solar masses.

- Up until now, scientists were unable to confirm that this elusive intermediate class actually existed.

- Farrell's team at the University of Leicester were analyzing archived data obtained by XMM-Newton, looking for neutron stars and white dwarves, when they stumbled upon a most peculiar object that was observed on 23 November 2004.

- Called HLX-1 (Hyper-Luminous X-ray source 1), it lies towards the outskirts of the galaxy ESO 243-49, approximately 290 million light-years from Earth. If it is indeed located in this distant galaxy, HLX-1 is very luminous in X-rays; peaking at 260 million times the luminosity of the Sun.

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Figure 67: Artist's impression of galaxy and HLX-1 (blue star to the left), image credit: Heidi Sagerud

Legend to Figure 67: Illustration of HLX-1 (blue star to the upper left hand side of the galactic bulge). HLX-1, located on the outskirts of the spiral galaxy ESO 243-49, is the strongest candidate to- date of intermediate-mass black holes.

• Nov. 14, 2008: X-ray and gamma-ray data from ESA's XMM-Newton and Integral orbiting observatories has been used to test, for the first time, the physical processes that make magnetars, an atypical class of neutron stars, shine in X-rays. 104)

- Neutron stars are remnants of massive stars (10-50 times as massive as our Sun) that have collapsed onto themselves under their own mass. Made almost entirely of neutrons (subatomic particles with no electric charge), these stellar corpses concentrate more than the mass of our Sun within a sphere about 20 km in diameter.

- They are so compact that a teaspoon of neutron star stuff would weigh about one hundred million tons. Two other physical properties characterize a neutron star: their fast rotation and strong magnetic field. - Magnetars form a class of neutron stars with ultra-strong magnetic fields. With magnetic fields a thousand times stronger than that of ordinary neutron stars, they are the strongest known magnets in the cosmos.

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Figure 68: Artist's impression of a magnetar (image credit: 2008 Sky & Telescope: Gregg Dinderman)

Legend to Figure 68: X-ray and gamma-ray data from ESA's XMM-Newton and Integral orbiting observatories has been used to test, for the first time, the physical processes thought to lie behind the emission of magnetars, an atypical class of neutron stars with ultra-strong magnetic fields.

• October 23, 2008: XMM-Newton, ESA's X-ray observatory, has re-established communication contact with Earth, showing that the spacecraft is safe and fully under control. The news was confirmed this morning by the mission control team at ESA's European Space Operations Center (ESOC) in Darmstadt, Germany. 105)

- Radio contact was re-established on Wednesday 22 October around 18:00 CEST (Central European Summer Time). This followed an unexpected radio silence from the spacecraft which started on 18 October 2008, when XMM-Newton, coming out of a nominal period of non-radio visibility along its orbit around Earth, did not succeed in sending the expected signal to Earth.

- Fearing the worst, teams at ESOC and other ESA establishments, supported by industry, immediately started studying the situation and performing the first attempts of recovery. The first signs of hope arrived on Monday evening when images obtained by the Starkenburg Observatory in Germany confirmed that the spacecraft was intact and did not give any sign that it was tumbling. This fact was also confirmed by many other ground observatories from all over the world that had answered XMM-Newton's call for help.

- The initial hopes were then confirmed, when ESOC ground controllers, using ESA's 35 m ground station in New Norcia (Western Australia), managed to establish a feeble two-way communication path to the spacecraft. This suggested a failure in the on-board Radio Frequency (RF) switch – an important clue for the experts that could then focus on more targeted solutions.

- Once these solutions were simulated and validated on ground, the final recovery action started. Ground controllers made use of the NASA 34 m DSN ( Deep Space Network) ground station in Goldstone (California), which is located in a favorable position for times when XMM-Newton is closest to Earth (perigee passage). They sent a command that allowed the RF switch to go back to its last working position and then managed to get radio contact with the spacecraft with ESA's 15 m ground station at ESA/ESAC (European Space and Astronomy Center) near Madrid, Spain.

- "XMM-Newton is now safe and fully under control," said Dietmar Heger, XMM-Newton Deputy Spacecraft Operations Manager at ESOC. "It's been a thrilling moment for our team. We even feared the spacecraft could be lost, but hard team work and a good star helped us turn this into a new success story for ESA."

- "XMM-Newton has been in orbit for almost nine years, investigating the violent Universe as never before and providing discovery after discovery" added Arvind Parmar, XMM-Newton Mission Manager at ESA/ESTEC (European Space Research and Technology Center) and Head of the Astronomy Science Operations Division at ESAC. "It's a big relief to know we can still count on this great spacecraft. We thank all the teams which have worked non-stop in the past days to make this possible, and our colleagues at NASA who made their 34 m station available."

• August 25, 2008: ESA's orbiting X-ray observatory XMM-Newton has discovered the most massive cluster of galaxies seen in the distant Universe until now. The galaxy cluster is so big that there can only be a handful of them at that distance, making this a rare catch indeed. The discovery confirms the existence of dark energy. 106)

- The newly-discovered monster, known only by the catalogue number 2XMM J083026+524133, is estimated to contain as much mass as a thousand large galaxies. Much of it is in the form of 100-million-degree hot gas. It was first observed by chance as XMM-Newton was studying another celestial object and 2XMM J083026+524133 was placed in a catalog for a future follow-up.

- Georg Lamer, AIP (Astrophysikalisches Institut Potsdam), Germany, and a team of astronomers discovered the record-breaking cluster as they were performing a systematic analysis of the catalogue. Based on 3500 observations performed with XMM-Newton's European Photon Imaging Camera (EPIC) covering about 1% of the entire sky, the catalogue contains more than 190 000 individual X-ray sources. The team were looking for extended patches of X-rays that could either be nearby galaxies or distant clusters of galaxies.

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Figure 69: 2XMM J083026+524133 distant cluster of galaxies (image credit: ESA XMM-Newton/EPIC, LBT/LBC, AIP, J. Kohnert)

The optical image that confirmed that 2XMM J083026+524133 is a distant cluster of galaxies, taken by the Large Binocular Telescope in Arizona. The X-ray emission from the cluster of galaxies is shown in blue at the center of the image. The individual galaxies in the cluster are the small dots inside the blue glow.

• June 23, 2008: XMM-Newton has, for the first time, detected signals from both stars of a binary pulsar system in X-rays, unveiling a scientific goldmine. Each star of the closely-packed system is a dense neutron star, spinning extremely fast, radiating X-rays in pulses. - The binary pulsar PSR J0737-3039 was first spotted by astronomers in 2003 in radio wavelengths. X-rays can be used to probe deeper and study the system more thoroughly. 107) 108)

- To see two pulsars orbiting each other in a binary system is extremely rare in itself. PSR J0737-3039 contains a slowly-rotating ‘lazy' neutron star (pulsar B) orbiting a faster and more energetic companion (pulsar A).

- Each pulsar or neutron star is the fast-rotating, dead heart of a once-massive star. "These stars are so dense that one cup of neutron star-stuff would outweigh Mt. Everest," says Alberto Pellizzoni, lead author of the article where the results are reported. "Add to that the fact that the two stars are orbiting really close to each other, separated by only 3 light-seconds, about three times the distance between Earth and the Moon."

- The fundamental physical processes involved in these extreme interactions are a matter of debate among theoretical physicists. But now, with XMM-Newton's observations, scientists have gained new insight, providing a new experimental setting for them. In X-rays, it will be possible to study the subsurface and magnetospheres of the stars as well as the interaction between the two in that close, heated environment.

• December 21, 2007: XMM-Newton has detected periodic X-ray emission, or the pulsed heartbeat of a weird new type of star. Collecting the X-rays from the so-called rotating radio transient has confirmed the nature of the underlying celestial object and given astronomers a new insight into these exotic objects. 109)

- The observations were made using XMM-Newton's EPIC (European Photon Imaging Camera), which targeted the celestial object RRAT J1819–1458. Astronomers observed the object for around 12 hours and detected pulsations in the X-ray data that show the source to be rotating once every 4.26 seconds.

- Previously, astronomers had only seen radio outbursts from this object. It erupts every three minutes or so with a brief burst of radio emission lasting just 3 ms (milliseconds). Such behavior defines the object as a RRAT (Rotating Radio Transient).

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Figure 70: An artist's impression of a rotating radio transient (image credit: Russel Kightly Media)

• Nov. 14, 2007: In recognition of their superb scientific output, the mission operations of European flagship x-ray and gamma-ray observatories, XMM-Newton and Integral, have been extended until 31 December 2012. The extension of both missions, which have been producing an incessant flow of science results since launch, was announced this week after a unanimous vote at a meeting of ESA's Science Program Committee. 110)

- In the last two years XMM-Newton has made breakthrough observations of a wide variety of compact objects, such as the first detection of an intermediate-mass black hole in globular cluster NGC 4472. This has direct implications for the formation and evolution theories of globular clusters in general.

- Thanks to its sensitivity at high energies, XMM-Newton has made the first and only direct probing of the central regions near a black hole, by sampling the presence of iron and the variability of its spectral fingerprints. The satellite's observations have also been fundamental in helping understand the physics of heavy sub-atomic matter (‘baryonic') in clusters of galaxies and on studying the dark matter component in clusters.

- XMM-Newton has given the first strong indication that very faint AGN (Active Galactic Nuclei) are similar to the 'normal' AGN population, and measured for the first time the size of the emission region of an AGN.

- Other major results include the progress in understanding the link between X-ray emission and luminosity of stars, as well as the relation between the X-ray emission and processes such as star accretion or collisions.

- The satellite has also discovered the remnants of a new class of supernovae within the so-called ‘Ia type', which are used as standard reference, or ‘candles', to determine stellar luminosity.

- XMM-Newton has also revealed X-ray emission in the Martian exosphere - the first definite detection of X-ray emission induced by exchange of electrical charges from the exosphere of another planet.

• Sept. 7, 2007: ESA released the largest catalog of X-ray sources ever, referred to as 2XMM, complied from 6 years of observations of the XMM-Newton mission. The catalog contains 247,000 X-ray source detections which relate to 192,000 unique X-ray sources, making it the largest collection of objects ever observed in the X-ray spectrum. 111)

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Figure 71: 2XMM compared to existing catalogs (image credit: University of Leicester, M. Watson) 112)

Legend to Figure 71: Covering 360 square degrees of the sky, 2XMM complements deeper Chandra and XMM-Newton small area studies and covers astronomical objects that dominate the X-ray background spread across the Universe. It provides a unique dataset for generating large, well-defined samples of various types of high-energy astrophysical objects. X-ray selection is the most efficient tool to pinpoint such objects.

XMM-Newton has, for the first time, detected signals from both stars of a binary pulsar system in X-rays, unveiling a scientific goldmine. Each star of the closely-packed system is a dense neutron star, spinning extremely fast, radiating X-rays in pulses. The binary pulsar PSR J0737-3039 was first spotted by astronomers in 2003 in radio wavelengths. XMM-Newton discovered X-ray emission from both pulsars in October 2006. 113)

• Nov. 27, 2001: Probably the most detailed analysis of the composition and dynamics of the supernova remnant Cassiopeia-A has been presented at the symposium 'New Visions of the X-ray Universe in the XMM-Newton and Chandra era' which is taking place this week at the ESA/ESTEC, Noordwijk in the Netherlands.

Cassiopeia A (Cas-A) is a young shell-shaped supernova remnant some 15 light years in diameter situated some 10,000 light years away. It is the remains of a massive star which, having exhausted all its hydrogen fuel, exploded 320 years ago. The core of such a collapsing star can give rise to a neutron star or black hole. Its external parts are blown apart projecting stellar material, glowing in X-rays, into the surrounding interstellar medium. The stellar material contains many heavy elements which have been forged from lighter elements in the progenitor star, and during the explosion process. All the chemical elements in our human bodies have their origins in such stellar explosions and the resulting primordial broth (Figure 72). 114)

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Figure 72: Abundance maps for the elements included in the Cassiopeia A spectral analysis of the data provided by XMM-Newton. All are plotted on the logarithmic scale indicated by the bar at the bottom (image credit: R. Willingale) 115)

• Feb. 9, 2000: The first pictures from ESA's new X-ray space observatory fully demonstrate the capabilities of the spacecraft's telescopes and its science instruments. The images were officially presented on 9 February at the XMM-Newton Science Operations Center in Villafranca, Spain. — The images were obtained between 19-25 January at the very start of the science payload commissioning process. The spacecraft viewed three regions of the sky: part of theLMC ( Large Magellanic Cloud ), the Hickson Cluster Group 16 (HCG-16), and the star HR 1099. These targets were chosen because they all present a variety of X-ray extended and point sources and are very interesting regions. 116)

The Large Magellanic Cloud is about 20 ,000 light years in diameter. Situated 160,000 light years from Earth, it is one of two irregularly shaped galaxies that are easily seen with the naked eye in the southern hemisphere. These galaxies are satellites of the Milky Way and appear to be slowly spiralling into our own Galaxy.

- The X-ray color image (Figure 73) made with the EPIC camera on XMM shows part of a small companion galaxy to the Milky Way, the Large Magellanic Cloud, where stellar explosions are releasing newly manufactured elements, and new stars are being formed. The image is made so as to reveal the temperature of the X-ray emitting medium, with blue indicating the hottest regions; green the intermediate temperatures and red the coldest regions. Most of the 'blue' X-rays have not been observed before, and it is the collecting power of XMM that enables these observations. 117)

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Figure 73: XMM-Newton EPIC-PN false color X-ray image of the 30 Doradus region in the Large Magellanic Cloud (image credit: ESA)

- XMM brings to bear the largest X-ray telescopes in space, and this is shown here. These colliding galaxies are 170 million light years away, and are clearly visible to EPIC. In the background more than a hundred faint X-ray sources are seen, most of these are new detections by XMM. 118)

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Figure 74: XMM-Newton EPIC-PN false color X-ray image of a group of HCG 16 (image credit: ESA)

- X-ray image of the field around the bright star HR1099 (Figure 75). This X-ray image also illustrates the collecting power of XMM, as many of the other X-ray sources in the image were hitherto unknown, or allow more detailed studies of these objects than before. HR1099 is a nearby star with a hot corona which is very active in X-rays. 119)

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Figure 75: EPIC-MOS X-ray image of the field around star HR1099 (image credit: ESA)

Minimize References
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68) "Bizarre nearby blast mimics Universe's most ancient stars," ESA, July 11, 2014, URL: http://m.esa.int/Our_Activities/Space_Science/Bizarre
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74) "Black hole boasts heavyweight jets," ESA, Nov. 13, 2013, URL: http://m.esa.int/Our_Activities/Space_Science/Black_hole_boasts_heavyweight_jets

75) María Díaz Trigo, James C. A. Miller-Jones, Simone Migliari, Jess W. Broderick, Tasso Tzioumis, "Baryons in the relativistic jets of the stellar-mass black-hole candidate 4U 1630-47," Nature, Vol. 504, pp: 260–262, 12 December 2013, doi:10.1038/nature12672

76) "Latest XMM-Newton Catalog offers new X-ray Vision," ESA, July 23, 2013, URL: http://sci.esa.int/xmm-newton/52082-latest-xmm
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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