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

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    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. 126).

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)

Figure 15: In this fifth episode of the Science@ESA vodcast series Rebecca Barnes will give us a glimpse of the hot, energetic and often violent Universe revealed through X-ray and gamma-ray astronomy, look at ESA missions that detect this hidden light and find out how the science that these missions perform is meticulously planned (video credit: ESA) 15)




Mission status:

• January 20, 2020: Material falling into a black hole casts X-rays out into space – and now, for the first time, ESA’s XMM-Newton X-ray observatory has used the reverberating echoes of this radiation to map the dynamic behavior and surroundings of a black hole itself. 16)

- Most black holes are too small on the sky for us to resolve their immediate environment, but we can still explore these mysterious objects by watching how matter behaves as it nears, and falls into, them.

- As material spirals towards a black hole, it is heated up and emits X-rays that, in turn, echo and reverberate as they interact with nearby gas. These regions of space are highly distorted and warped due to the extreme nature and crushingly strong gravity of the black hole.

- For the first time, researchers have used XMM-Newton to track these light echoes and map the surroundings of the black hole at the core of an active galaxy. Named IRAS 13224–3809, the black hole’s host galaxy is one of the most variable X-ray sources in the sky, undergoing very large and rapid fluctuations in brightness of a factor of 50 in mere hours.

- “Everyone is familiar with how the echo of their voice sounds different when speaking in a classroom compared to a cathedral – this is simply due to the geometry and materials of the rooms, which causes sound to behave and bounce around differently,” explains William Alston of the University of Cambridge, UK, lead author of the new study. 17)

- “In a similar manner, we can watch how echoes of X-ray radiation propagate in the vicinity of a black hole in order to map out the geometry of a region and the state of a clump of matter before it disappears into the singularity. It’s a bit like cosmic echo-location.”

Figure 16: These illustrations show the surroundings of a black hole feeding on ambient gas as mapped using ESA’s XMM-Newton X-ray observatory. As the material falls into the black hole, it spirals around to form a flattened disc, as shown here, heating up as it does so. At the very center of the disc, close to the black hole, a region of very hot electrons – with temperatures of around a billion degrees – known as the corona produced high-energy X-rays that stream out into space. 18)

- As the dynamics of infalling gas are strongly linked to the properties of the consuming black hole, William and colleagues were also able to determine the mass and spin of the galaxy’s central black hole by observing the properties of matter as it spiralled inwards.

- The inspiralling material forms a disc as it falls into the black hole. Above this disc lies a region of very hot electrons – with temperatures of around a billion degrees – called the corona. While the scientists expected to see the reverberation echoes they used to map the region’s geometry, they also spotted something unexpected: the corona itself changed in size incredibly quickly, over a matter of days.

- “As the corona’s size changes, so does the light echo – a bit like if the cathedral ceiling is moving up and down, changing how the echo of your voice sounds,” adds William.

- “By tracking the light echoes, we were able to track this changing corona, and – what’s even more exciting – get much better values for the black hole’s mass and spin than we could have determined if the corona was not changing in size. We know the black hole's mass cannot be fluctuating, so any changes in the echo must be down to the gaseous environment.”

- The study used the longest observation of an accreting black hole ever taken with XMM-Newton, collected over 16 spacecraft orbits in 2011 and 2016 and totalling 2 million seconds – just over 23 days.

- This, combined with the strong and short-term variability of the black hole itself, allowed William and collaborators to model the echoes comprehensively over day-long timescales.

- The region explored in this study is not accessible to observatories such as the Event Horizon Telescope, which managed to take the first ever picture of gas in the immediate vicinity of a black hole – the one sitting at the center of the nearby massive galaxy M87. The result, based on observations performed with radio telescopes across the world in 2017 and published last year, immediately became a global sensation.

- “The Event Horizon Telescope image was obtained using a method known as interferometry – a wonderful technique that can only work on the very few nearest supermassive black holes to Earth, such as those in M87 and in our home galaxy, the Milky Way, because their apparent size on the sky is large enough for this method to work,” says co-author Michael Parker, who is an ESA research fellow at the European Space Astronomy Center near Madrid, Spain.

- “By contrast, our approach is able to probe the nearest few hundred supermassive black holes that are actively consuming matter – and this number will increase significantly with the launch of ESA’s Athena satellite.”

- Characterizing the environments closely surrounding black holes is a core science goal for ESA’s Athena mission, which is scheduled for launch in the early 2030s and will unveil the secrets of the hot and energetic Universe.

- Measuring the mass, spin and accretion rates of a large sample of black holes is key to understanding gravity throughout the cosmos.

- Additionally, since supermassive black holes are strongly linked to their host galaxy’s properties, these studies are also key to furthering our knowledge of how galaxies form and evolve over time.

- “The large dataset provided by XMM-Newton was essential for this result,” says Norbert Schartel, ESA XMM-Newton Project Scientist.

- “Reverberation mapping is an exciting technique that promises to reveal much about both black holes and the wider Universe in coming years. I hope that XMM-Newton will perform similar observing campaigns for several more active galaxies in coming years, so that the method is fully established when Athena launches.”

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Figure 17: The core of massive galaxy M87 as viewed in X-rays by ESA’s XMM-Newton space observatory. A giant elliptical galaxy, M87 is home to several trillion stars, making it one of the most massive galaxies in the local Universe. About 52 million light years away, it is located at the center of the Virgo cluster, the nearest cluster of galaxies to the Local Group, to which our own Milky Way galaxy belongs. A supermassive black hole as massive as billions of stars like our Sun sits at the core of M87, accreting material from its surroundings at an extremely intense rate. The black hole’s accretion produces powerful jets that launch energetic particles close to the speed of light outwards into the surrounding cluster environment, as well as inflating giant bubbles that lift cooler gas from the cluster center and form the filamentary structures visible in this image. The activity of the black hole also generates shock waves, such as the circular feature that can be seen around the center of the image. This view is based on data collected at X-ray energies between 0.3 and 7 keV with the EPIC (European Photon Imaging Camera) onboard XMM-Newton on 16 July 2017. The image spans 40 arcminutes on each side (image credit: ESA/XMM-Newton; Acknowledgement: P. Rodriguez)

• January 16, 2020: ESA’s XMM-Newton has discovered that gas lurking within the Milky Way’s halo reaches far hotter temperatures than previously thought and has a different chemical make-up than predicted, challenging our understanding of our galactic home. 19)

- A halo is a vast region of gas, stars and invisible dark matter surrounding a galaxy. It is a key component of a galaxy, connecting it to wider intergalactic space, and is thus thought to play an important role in galactic evolution.

- Until now, a galaxy’s halo was thought to contain hot gas at a single temperature, with the exact temperature of this gas dependent on the mass of the galaxy.

- However, a new study using ESA’s XMM-Newton X-ray space observatory now shows that the Milky Way’s halo contains not one but three different components of hot gas, with the hottest of these being a factor of ten hotter than previously thought. This is the first time multiple gas components structured in this way have been discovered in not only the Milky Way, but in any galaxy. 20)

- “We thought that gas temperatures in galactic haloes ranged from around 10,000 to one million degrees – but it turns out that some of the gas in the Milky Way’s halo can hit a scorching 10 million degrees,” says Sanskriti Das, a graduate student at The Ohio State University, USA, and lead author of the new study.

- “While we think that gas gets heated to around one million degrees as a galaxy initially forms, we’re not sure how this component got so hot. It may be due to winds emanating from the disc of stars within the Milky Way.”

- The study used a combination of two instruments aboard XMM-Newton: the Reflection Grating Spectrometer (RGS) and European Photon Imaging Camera (EPIC). EPIC was used to study the light emitted by the halo, and RGS to study how the halo affects and absorbs light that passes through it.

- To probe the Milky Way’s halo in absorption, Sanskriti and colleagues observed an object known as a blazar: the very active, energetic core of a distant galaxy that is emitting intense beams of light.

- Having travelled almost five billion light-years across the cosmos, the X-ray light from this blazar also passed through our galaxy’s halo before reaching XMM-Newton’s detectors, and thus holds clues about the properties of this gaseous region.

- Unlike previous X-ray studies of the Milky Way’s halo, which normally last a day or two, the team performed observations over a period of three weeks, enabling them to detect signals that are usually too faint to see.

- “We analyzed the blazar’s light and zeroed in on its individual spectral signatures: the characteristics of the light that can tell us about the material it’s passed through on its way to us,” says co-author Smita Mathur, also of The Ohio State University, and Sanskriti’s advisor.

- “There are specific signatures that only exist at specific temperatures, so we were able to determine how hot the halo gas must have been to affect the blazar light as it did.”

- The Milky Way’s hot halo is also significantly enhanced with elements heavier than helium, which are usually produced in the later stages of a star’s life. This indicates that the halo has received material created by certain stars during their lifetimes and final stages, and flung out into space as they die.

Figure 18: This animated artist's impression shows the Milky Way (the small galaxy depicted at the center of the frame) and its halo (the extended gaseous region). It illustrates the halo in three different shades – emerald, yellow and green. These all mix together throughout the halo, and each represents gas of a different temperature. Dots then appear across this halo; these represent elements and their relative abundances, as detected by ESA’s XMM-Newton X-ray space observatory: nitrogen (black, 41 dots), neon (orange/yellow, 39 dots), oxygen (light blue, 7 dots) and iron (red, 1 dot). - A halo is a vast region of gas, stars and invisible dark matter surrounding a galaxy. It is a key component of a galaxy, connecting it to wider intergalactic space, and is thus thought to play an important role in galactic evolution (image credit: ESA)

- A study using XMM-Newton now shows that the Milky Way’s halo contains not one but three different components of hot gas, with the hottest of these being a factor of ten hotter than previously thought. This is the first time multiple gas components structured in this way have been discovered in not only the Milky Way, but in any galaxy.

- A study using XMM-Newton now shows that the Milky Way’s halo contains not one but three different components of hot gas, with the hottest of these being a factor of ten hotter than previously thought. This is the first time multiple gas components structured in this way have been discovered in not only the Milky Way, but in any galaxy.

- “Until now, scientists have primarily looked for oxygen, as it’s abundant and thus easier to find than other elements,” explains Sanskriti.

- “Our study was more detailed: we looked at not only oxygen but also nitrogen, neon and iron, and found some hugely interesting results.”

- Scientists expect the halo to contain elements in similar ratios to those seen in the Sun. However, Das and colleagues noticed less iron in the halo than expected, indicating that the halo has been enriched by massive dying stars, and also less oxygen, likely due to this element being taken up by dusty particles in the halo.

- “This is really exciting – it was completely unexpected, and tells us that we have much to learn about how the Milky Way has evolved into the galaxy it is today,” adds Sanskriti.

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Figure 19: The cosmic budget of ‘ordinary’ matter. While the mysterious dark matter and dark energy make up about 25 and 70 percent of our cosmos respectively, the ordinary matter that makes up everything we see – from stars and galaxies to planets and people – amounts to only about five percent. — However, stars in galaxies across the Universe only make up about seven percent of all ordinary matter. The cold interstellar gas that permeates galaxies – the raw material to create stars – amounts to about 1.8 percent of total, while the hot, diffuse gas in the haloes that encompass galaxies makes up roughly five percent, and the even hotter gas that fills galaxy clusters – the largest cosmic structures held together by gravity – accounts for four percent. — This is not surprising: stars, galaxies and galaxy clusters form in the densest knots of the cosmic web, the filamentary distribution of both dark and ordinary matter that extends throughout the Universe. While these sites are dense, they are also rare, so not the best spots to look for the majority of cosmic matter. — Most of the Universe’s ordinary matter, or baryons, must be lurking in the ubiquitous filaments of this cosmic web, where matter is however less dense and therefore more challenging to observe. Using different techniques over the years, they were able to locate a good chunk of this intergalactic material – mainly its cool component (also known as Lyman-alpha forest, which makes up about 28 percent of all baryons) and its warm component (about 15 percent). — After two decades of observations, astronomers using ESA’s XMM-Newton space observatory have detected the hot component of this intergalactic material along the line of sight to a distant quasar. The amount of hot intergalactic gas detected in these observations amounts up to 40 percent of all baryons in the Universe, closing the gap in the overall budget of ordinary matter in the cosmos (image credit: ESA)

- The newly discovered hot gas component also has wider implications that affect our overall understanding of the cosmos. Our galaxy contains far less mass than we expect: this is known as the ‘missing matter problem’, in that what we observe does not match up with theoretical predictions.

- From its long-term mapping of the cosmos, ESA’s Planck spacecraft predicted that just under 5% of the mass in the Universe should exist in the form of ‘normal’ matter – the kind making up stars, galaxies, planets, and so on.

- “However, when we add up everything we see, our figure is nowhere near this prediction,” adds co-author Fabrizio Nicastro of Osservatorio Astronomico di Roma—INAF, Italy, and the Harvard-Smithsonian Center for Astrophysics, USA.

- “So where’s the rest? Some suggest that it may be hiding in the extended and massive halos surrounding galaxies, making our finding really exciting.”

- As this hot component of the Milky Way’s halo has never been seen before, it may have been overlooked in previous analyses – and may thus contain a large amount of this ‘missing’ matter.

- “These observations provide new insights into the thermal and chemical history of the Milky Way and its halo, and challenge our knowledge of how galaxies form and evolve,” concludes ESA XMM project scientist Norbert Schartel.

- “The study looked at the halo along one sightline – that towards the blazar – so it will be hugely exciting to see future research expand on this.”

• January 10, 2020: Using XMM-Newton to study Perseus, astronomers spotted the first signs of this hot gas splashing and sloshing around – a behavior that, while predicted, had never been seen before. 21)

- ESA’s XMM-Newton X-ray observatory has spied hot gas sloshing around within a galaxy cluster – a never-before-seen behavior that may be driven by turbulent merger events.

- Galaxy clusters are the largest systems in the Universe bound together by gravity. They contain hundreds to thousands of galaxies and large quantities of hot gas known as plasma, which reaches temperatures of around 50 million degrees and shines brightly in X-rays.

- Very little is known about how this plasma moves, but exploring its motions may be key to understanding how galaxy clusters form, evolve and behave.

- “We selected two nearby, massive, bright and well-observed galaxy clusters, Perseus and Coma, and mapped how their plasma moved – whether it was moving towards or away from us, its speed, and so on – for the first time,” says Jeremy Sanders of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and lead author of the new study. 22)

- “We did this over large regions of sky: an area roughly the size of two full Moons for Perseus, and four for Coma. We really needed XMM-Newton for this, as it’d be extremely difficult to cover such large areas with any other spacecraft.”

- Jeremy and colleagues found direct signs of plasma flowing, splashing and sloshing around within the Perseus galaxy cluster – one of the most massive known objects in the Universe, and the brightest cluster in the sky in terms of X-rays. While this kind of motion has been predicted theoretically, it had never been seen before in the cosmos.

- By looking at simulations of how the plasma moved within the cluster, the researchers then explored what was causing the sloshing. They found it to be likely due to smaller sub-clusters of galaxies colliding and merging with the main cluster itself. These events are energetic enough to disrupt Perseus’ gravitational field and kickstart a sloshing motion that will last for many millions of years before settling.

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Figure 20: This image shows the Perseus galaxy cluster – one of the most massive known objects in the Universe – in X-ray and optical light, as seen by XMM-Newton’s European Photon Imaging Camera (EPIC) and the Digitized Sky Survey II, respectively (image credit: ESA/XMM-Newton/SDSS/J. Sanders et al. 2019)

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Figure 21: Gas motions in the Perseus cluster. This image shows the Perseus galaxy cluster – one of the most massive known objects in the Universe – in X-rays, as seen by XMM-Newton’s EPIC (European Photon Imaging Camera). The central region of the cluster can be seen glowing brightly, with its diffuse outer regions extending outwards from the middle of the frame. Perseus’ density rises quickly as one approaches the cluster’s cool center; this is reflected in its X-ray brightness, which changes rapidly with radius (as illustrated by the coloring of this image). The overlaid blue and red arrows show the motion of the gas in the region (relative to the cluster itself), with blue arrows representing gas moving towards us, and red representing gas moving away. The length of the ‘tail’ on the arrows represents the size of the velocity: the longer the arrow tail, the faster the gas is moving. This image is part of a new study of Perseus that spotted the first signs of this gas splashing and sloshing around – a behavior that, while predicted, had never been seen before (image credit: ESA/XMM-Newton/J. Sanders et al. 2019)

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Figure 22: This image shows a simulation of the Perseus galaxy cluster – one of the most massive known objects in the Universe. A new study based on observations from ESA’s XMM-Newton has spotted the first signs of this gas splashing and sloshing around within Perseus – a behavior that, while predicted, had never been seen before. These three simulated frames show the surface brightness of the cluster gas in X-rays (left), the temperature of the gas (middle), and the velocity of the gas (right). The brightest gas is seen in shades of orange-yellow and the dimmest in dark purple and black (left); the hottest gas is in dark red and the coolest in dark blue (center); and the fastest gas is in shades of lime and yellow, while the slowest is dark blue (right), image credit: Courtesy of J. Hone, Harvard-Smithsonian Center for Astrophysics

- Unlike Perseus, which is characterized by a main cluster and several smaller sub-structures, the Coma cluster contained no sloshing plasma, and appears to instead be a massive cluster made up of two major sub-clusters that are slowly merging together.

- “Coma contains two massive central galaxies rather than a cluster’s usual single behemoth, and different regions appear to contain material that moves differently,” says Jeremy.

- “This indicates that there are multiple streams of material within the Coma cluster that haven’t yet come together to form a single coherent ‘blob’, like we see with Perseus.”

- The finding was made possible by a new calibration technique applied to XMM-Newton’s European Photon Imaging Camera (EPIC). The ingenious method, which involved mining two decades of archival EPIC data, improved the accuracy of the camera’s velocity measurements by a factor of over 3.5, raising XMM-Newton’s capabilities to a new level.

- “The EPIC camera has an instrumental background signal – the so-called ‘fluorescent lines’ which are always present in our data, and can sometimes be annoying as they’re usually not what we’re looking for,” adds co-author Ciro Pinto, an ESA research fellow at ESTEC (European Space Research and Technology Center) in Noordwijk, The Netherlands, who recently moved to Italy’s National Institute for Astrophysics.

- “We decided to use these lines, which are a constant feature, to compare and align EPIC data from the past 20 years to better determine how the camera behaves, and then used this to correct for any instrumental variation or effects.”

- This technique made it possible to map the gas in the clusters more accurately. Jeremy, Ciro and colleagues used the background lines to recognize and remove individual variations between observations, and then eliminated any subtler instrumental effects identified and flagged up by their 20 years of EPIC data mining.

- EPIC comprises three CCD cameras designed to capture both low- and high-energy X-rays, and is one of a trio of advanced instruments aboard XMM-Newton.

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Figure 23: This image shows the bright, nearby, and massive Coma galaxy cluster in X-ray and optical light, as seen by XMM-Newton’s EPIC and the SDSS (Sloan Digital Sky Survey), image credit: ESA/XMM-Newton/SDSS/J. Sanders et al. 2019

- Exploring the dynamic X-ray sky since its launch in 1999, XMM-Newton is the biggest scientific satellite ever built in Europe, and carries some of the most powerful telescope mirrors ever developed.

- “This calibration technique highlights newfound capabilities of the EPIC camera,” says Norbert Schartel, ESA XMM-Newton Project Scientist.

- “High-energy astrophysics often entails comparing X-ray data at different points in the cosmos for everything from plasma to black holes, so the ability to minimize instrumental effects is key. By using past XMM-Newton observations to refine future ones, the new technique may open up inspiring opportunities for new research and discovery.”

- These XMM-Newton observations will also remain unparalleled until the launch of ESA’s Advanced Telescope for High-ENergy Astrophysics (ATHENA) in 2031. Whereas covering such large areas of sky will largely be beyond the capabilities of telescopes such as the upcoming JAXA/NASA XRISM (X-ray Imaging and Spectroscopy Mission), Athena will combine a large X-ray telescope with state-of-the-art scientific instruments to shed new light on the hot, energetic Universe.

• On 10 December 2019, ESA’s XMM-Newton X-ray space observatory is celebrating its 20th launch anniversary. In those two decades, the observatory has supplied a constant stream of outstanding science. One area that the mission has excelled in is the science of black holes, having had a profound effect on our understanding of these cosmic enigmas. 23)

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Figure 24: Black holes are celestial objects so dense that nothing, not even light, can escape their pull. In this artist’s impression, the weird shapes of light around the black hole are what computer simulations predict will happen in the vicinity of its intense gravitational field (image credit: ESA/XMM-Newton/I. de la Calle)

- Although neither XMM-Newton nor any other telescope can actually see black holes in this detail, the mission’s data and observations have provided a great source of information about these mysterious gravitational traps. In particular, XMM-Newton has been particularly good at isolating the X-rays given out by high-temperature, ionized atoms of iron as they swirl towards doom in the black hole.

- The X-rays given out from the iron contain information about the geometry and dynamics of the black hole. In 2013, XMM-Newton was used to measure such emission in order to study the rotation rate of the supermassive black hole at the center of the spiral galaxy NGC 1365.

- Supermassive black holes, with masses between millions and billions of times the mass of our Sun, are thought to lurk in the center of almost every large galaxy in the Universe. Their rotation rate is important because it can give away important details about the history of their host galaxy.

- A fast rotating black hole is fed by a uniform stream of matter falling together, or by galaxies merging with one another, whereas a slowly rotating black hole is buffeted from all sides by small clumps of matter hitting it. In the case of NGC 1365, XMM-Newton showed that the black hole was rotating quickly and so the galaxy probably grew steadily over time, or merged with others.

- More recently, XMM-Newton discovered mysterious flashes from the black hole at the center of another galaxy called GSN 069. These flares took place every nine hours or so, raising the brightness of the X-ray emission by a factor of 100. These eruptions are thought to be coming from the matter caught in the black hole’s gravitational grip or from a less massive black hole circling the more massive one.

- As XMM-Newton continues into its third decade, black holes and the galaxies they are found in will continue to be a priority target.

• September 11, 2019: ESA’s X-ray space telescope XMM-Newton has detected never-before-seen periodic flares of X-ray radiation coming from a distant galaxy that could help explain some enigmatic behaviors of active black holes. 24)

- XMM-Newton, the most powerful X-ray observatory, discovered some mysterious flashes from the active black hole at the core of the galaxy GSN 069, about 250 million light years away. On 24 December 2018, the source was seen to suddenly increase its brightness by up to a factor 100, then dimmed back to its normal levels within one hour and lit up again nine hours later.

Figure 25: An X-ray view of the active black hole at the core of distant galaxy GSN 069, about 250 million light years away, based on data from ESA’s XMM-Newton X-ray observatory. The upper part of the animation shows the actual observations, and the graph in the lower part shows variations of the X-ray brightness of the source relative to its ‘dormant’ level. - This animation is based on nearly 40 hours of observations of this source, which undergoes never-before-seen flashes – dubbed QPEs (Quasi-Periodic Eruptions) every nine hours. The sequence has been speeded up for illustration purposes; each frame corresponds to about three minutes of actual XMM-Newton exposure time [image credit: ESA/XMM-Newton; G. Miniutti & M. Giustini (CAB, CSIC-INTA, Spain)]

- “It was completely unexpected,” says Giovanni Miniutti, of the Centro de Astrobiología in Madrid, Spain, lead author of a new paper published in the journal Nature today. 25)

- “Giant black holes regularly flicker like a candle but the rapid, repeating changes seen in GSN 069 from December onwards are something completely new.”

- Further observations, performed with XMM-Newton as well as NASA’s Chandra X-ray observatory in the following couple of months, confirmed that the distant black hole was still keeping the tempo, emitting nearly periodic bursts of X-rays every nine hours. The researchers are calling the new phenomenon QPEs.

- “The X-ray emission comes from material that is being accreted into the black hole and heats up in the process,” explains Giovanni.

- “There are various mechanisms in the accretion disc that could give rise to this type of quasi-periodic signal, potentially linked to instabilities in the accretion flow close to the central black hole.

- “Alternatively, the eruptions could be due to the interaction of the disc material with a second body – another black hole or perhaps the remnant of a star previously disrupted by the black hole.”

- Although never before observed, Giovanni and colleagues think periodic flares like these might actually be quite common in the Universe.

Figure 26: Optical and X-ray view of active galaxy GSN 069. The main panel of this graphic is a visible light image taken by the DSS (Digitized Sky Survey) around the galaxy known as GSN 069. The inset gives a time-lapse of data taken by NASA’s Chandra X-ray observatory over a period of about 20 hours on 14 and 15 February 2019. The sequence loops over again to show how the X-ray brightness of the source in the center of GSN 069 regularly changes dramatically over that span (image credit: X-ray: NASA/CXO/CSIC-INTA/G. Miniutti et al.; Optical: DSS)

- It is possible that the phenomenon had not been identified before because most black holes at the cores of distant galaxies, with masses millions to billions of times the mass of our Sun, are much larger than the one in GSN 069, which is only about 400,000 times more massive than our Sun.

- The bigger and more massive the black hole, the slower the fluctuations in brightness it can display, so a typical supermassive black hole would erupt not every nine hours, but every few months or years. This would make detection unlikely as observations rarely span such long periods of time.

- And there is more. Quasi-periodic eruptions like those found in GSN 069 could provide a natural framework to interpret some puzzling patterns observed in a significant fraction of active black holes, whose brightness seems to vary too fast to be easily explained by current theoretical models.

- “We know of many massive black holes whose brightness rises or decays by very large factors within days or months, while we would expect them to vary at a much slower pace,” says Giovanni.

- “But if some of this variability corresponds to the rise or decay phases of eruptions similar to those discovered in GSN 069, then the fast variability of these systems, which appears currently unfeasible, could naturally be accounted for. New data and further studies will tell if this analogy really holds.”

- The quasi-periodic eruptions spotted in GSN 069 could also explain another intriguing property observed in the X-ray emission from nearly all bright, accreting supermassive black holes: the so-called ‘soft excess’.

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Figure 27: X-ray flares from active galaxy GSN 069. Variations in the X-ray brightness of the active black hole at the core of distant galaxy GSN 069, about 250 million light years away, as recorded by ESA’s XMM-Newton X-ray observatory (blue) and NASA’s Chandra X-ray observatory (red). The graph shows the X-ray brightness of the source relative to its ‘dormant’ level. This source was first observed to undergo before-seen flashes on 24 December 2018, when its brightness suddenly increased by up to a factor 100, then dimmed back to its normal levels within one hour and lit up again nine hours later. Further observations performed over a period of 54 days confirmed this flaring behavior, with ‘quasi-periodic eruptions’, or QPEs detected every nine hours [image credit: ESA/XMM-Newton; NASA/CXC; G. Miniutti (CAB, CSIC-INTA, Spain)]

- It consists in enhanced emission at low X-ray energies, and there is still no consensus on what causes it, with one leading theory invoking a cloud of electrons heated up near the accretion disc.

- Like similar black holes, GSN 069 exhibits such a soft X-ray excess during bursts, but not between eruptions.

- “We may be witnessing the formation of the soft excess in real time, which could shed light on its physical origin,” says co-author Richard Saxton from the XMM-Newton operation team at ESA’s astronomy center in Spain.

- “How the cloud of electrons is created is currently unclear, but we are trying to identify the mechanism by studying the changes in the X-ray spectrum of GSN 069 during the eruptions.”

- The team is already trying to pinpoint the defining properties of GSN 069 at the time when the periodic eruptions were first detected to look for more cases to study.

- "One of our immediate goals is to search for X-ray quasi-periodic eruptions in other galaxies, to further understand the physical origin of this new phenomenon,” adds co-author Margherita Giustini of Madrid’s Centro de Astrobiología.

- “GSN 069 is an extremely fascinating source, with the potential to become a reference in the field of black hole accretion,” says Norbert Schartel, ESA’s XMM-Newton project scientist.

- The discovery would not have been possible without XMM-Newton’s capabilities.

- “These bursts happen in the low energy part of the X-ray band, where XMM-Newton is unbeatable. We will certainly need to use the observatory again if we want to find more of these kinds of events in the future,” concludes Norbert.

• August 26, 2019: This colorful spread of light specks is in fact a record of extremely powerful phenomena taking place in a galaxy known as Messier 83, or M83. Located some 15 million light-years away, M83 is a barred spiral galaxy, not dissimilar in shape from our own Milky Way, and currently undergoing a spur of star formation, with a handful of new stars being born every year. 26)


Figure 28: This animation shows an X-ray view of the spiral galaxy Messier 83, based on data from ESA's XMM-Newton space observatory. The data were gathered on six occasions – January 2003, January and August 2014, February and August 2015, and January 2016 – at energies of 0.2–2 keV (shown in red), 2–4.5 keV (shown in green), and 4.5–12 keV (shown in blue), image credit: ESA/XMM-Newton – Acknowledgement: S. Carpano, Max-Planck Institute for Extraterrestrial Physics

- Most of the dots in this view represent the end points of the life cycle of stars, including remnants of supernova explosions and binary systems featuring compact stellar remnants like neutron stars or black holes that are feeding on matter from a companion star. In particular, the large speck to the lower left of the galaxy’s central region is what astronomers call an ultra-luminous X-ray source, or ULX, a binary system where the compact remnant is accreting mass from its companion at a much higher rate than an ordinary X-ray binary.

- The highly energetic phenomena that can be observed with X-ray telescopes often undergo regular changes, on time scales of days or even hours, turning the X-ray sky into a spectacular light show.

- The sources located in the reddish area at the center of the image correspond to objects located in the inner portions of M83. The majority of sources scattered across the image are located in the outskirts of the galaxy, but a few of those are foreground stars in our own galaxy, and others correspond to more distant galaxies in the background. 27)

• July 8, 2019: While observing the sky in X-rays, ESA’s XMM-Newton spots thousands and thousands of serendipitous sources. The catalog, released in May 2018, features sources in the 0.2 to 12 keV energy range drawn from 10,242 observations made by XMM-Newton’s European Photon Imaging Camera (EPIC), an instrument capable of detecting very faint sources and rapid changes in intensity, between 3 February 2000 and 30 November 2017. It contains 532 more observations and 47,363 more detections than the preceding 3XMM-DR7 catalog, which was made public in June 2017. 28)

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Figure 29: The purple lines and blotches scattered across this image show something incredible: all of the X-ray sources that were serendipitously detected – that is, not intentionally targeted – by ESA’s XMM-Newton X-ray space observatory from 2000 to 2017. This image is based on a catalog named 3XMM-DR8, the latest publicly released catalog of serendipitous XMM-Newton X-ray sources, created on behalf of ESA by the XMM-Newton Survey Science Center (image credit: ESA/XMM-Newton/N. Webb (XMM-Newton Survey Science Center), CC BY-SA 3.0 IGO)

- While the pattern of sources across the sky may appear random, some structure can be seen here. The oval represents the celestial sphere, an abstract perspective upon which our observations of the Universe are projected. The data are plotted in galactic coordinates, such that the center of the plot corresponds to the center of our Milky Way galaxy – and this can be seen in the image. Through the center of the oval is a horizontal line, where patches of purple appear to draw together. This line is the plane of the Milky Way galaxy, with the large splotch of color in the center corresponding to our galaxy’s core, where XMM-Newton made a higher number of serendipitous detections.

- XMM-Newton has been orbiting the Earth since 1999, observing the cosmos around us while on the hunt for X-rays coming from high-energy phenomena such as black holes, stellar winds, pulsars, and neutron stars. With every patch of sky that XMM-Newton observes, the telescope detects between 50 and 100 serendipitous sources, such as those shown here, besides the objects that were the original target of the observations. This is due to the large collecting area of the telescope’s mirrors and its wide field of view.

- All-sky images and large-scale cosmic data are immensely valuable in our study of the cosmos. Upcoming missions – such as the eROSITA space telescope, a German-led satellite scheduled for launch on 12 July 2019 to complete the first all-sky survey in the medium-energy X-ray band, up to 10 keV – will add to this wealth of knowledge, and help further our understanding of the X-ray Universe.

• May 6, 2019: The image of Figure 30 shows a quasar nicknamed the Teacup due to its shape. A quasar is an active galaxy that is powered by material falling into its central supermassive black hole. They are extremely luminous objects located at great distances from Earth. The Teacup is 1.1 billion light years away and was thought to be a dying quasar until recent X-ray observations shed new light on it. 29) 30)

- The Teacup was discovered in 2007 as part of the Galaxy Zoo project, a citizen science project that classified galaxies using data from the Sloan Digital Sky Survey. A powerful eruption of energy and particles from the central black hole created a bubble of material that became the Teacup's handle, which lies around 30,000 light years from the center.

- Observations revealed ionized atoms in the handle of the Teacup, possibly caused by strong radiation coming from the quasar in the past. This past level of radiation dwarfed the current measurements of the luminosity from the quasar. The radiation seemed to have diminished by 50 to 600 times over the last 40,000 to 100,000 years, leading to the theory that the quasar was rapidly fading.

- But new data from ESA's XMM-Newton telescope and NASA's Chandra X-ray observatory reveal that X-rays are coming from a heavily obscured central source, which suggests that the quasar is still burning bright beneath its shroud. While the quasar has certainly dimmed over time, it is nowhere near as significant as originally thought, perhaps only fading by a factor of 25 or less over the past 100,000 years.

- The Chandra data also showed evidence for hotter gas within the central bubble, and close to the 'cup' which surrounds the central black hole. This suggests that a wind of material is blowing away from the black hole, creating the teacup shape.

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Figure 30: In the image shown here the X-ray data is colored in blue and optical observations from the NASA/ESA Hubble Space Telescope are shown in red and green. Another image including radio data also shows a second ‘handle’ on the other side of the 'cup' (image credit: X-ray: NASA/CXC/University of Cambridge/G. Lansbury et al; optical: NASA/STScI/W. Keel et al.)

• April 10, 2019: A giant elliptical galaxy, M87 is home to several trillion stars, making it one of the most massive galaxies in the local Universe. About 52 million light years away, it is located at the center of the Virgo cluster, the nearest cluster of galaxies to the Local Group, to which our own Milky Way galaxy belongs. 31)

- A supermassive black hole as massive as billions of stars like our Sun sits at the core of M87, accreting material from its surroundings at an extremely intense rate. The black hole’s accretion produces powerful jets that launch energetic particles close to the speed of light outwards into the surrounding cluster environment, as well as inflating giant bubbles that lift cooler gas from the cluster center and form the filamentary structures visible in this image.

- On 10 April 2019, the EHT (Event Horizon Telescope) – a planet-scale array of eight ground-based radio telescopes forged through international collaboration – presented the first direct visual evidence of a supermassive black hole and its shadow: the black hole at the core of M87. The EHT observations were also performed in 2017.

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Figure 31: The core of the massive galaxy M87 (Messier 87) as viewed in X-rays by ESA’s XMM-Newton space observatory. The activity of the black hole also generates shock waves, such as the circular feature that can be seen around the center of the image. This view is based on data collected at X-ray energies between 0.3 and 7 keV with the EPIC camera onboard XMM-Newton on 16 July 2017. The image spans 40 arcminutes on each side (image credit: ESA/XMM-Newton; Acknowledgement: P. Rodriguez)

• April 8, 2019: An X-ray machine which uses space technology to generate crystal clear images that doctors can use to detect the early signs of cancer has been prioritized for €1.2 m of funding by the European Space Agency and the UK Space Agency. 32)

- Cancers are often missed on normal X-rays, which produce slightly fuzzy images that can be difficult to interpret. This can mean the disease is more advanced and difficult to treat by the time it is discovered.

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Figure 32: Concept system of a 3D mobile X-ray machine (image credit: Adaptix Imaging)

- So engineers from the UK company Adaptix have used technology developed for space to produce three-dimensional scans that generate much clearer images.

- The device employs X-ray optics deployed on spacecraft such as ESA’s XMM-Newton mission, which launched in 1999 and is observing stars at X-ray wavelengths.

- Miniaturized, portable and connected through satellites, the machine should also allow patients to be scanned in GPs’ surgeries, reducing the need for trips to hospital and shortening waiting times for patients.

- Tony Young, national clinical director for innovation at NHS (National Health Service) England, said: “Last year as we celebrated the NHS’s 70th birthday, we challenged industry to bring technology designed for outer space into the NHS. Using stargazing technology to spot cancer is exactly the type of advanced innovation that could improve care for patients by speeding up diagnosis and helping to deliver our long-term plan which will save half a million lives.”

- Chris Skidmore, the UK science minister, said: “The challenge of working in space focuses some of the UK’s most brilliant minds. These experts can also help transform our lives for the better here on Earth.

- “The huge potential of space technology isn’t just about reaching out into the universe—it’s here on earth that its greatest impact can be seen, from 5G to tackling climate change or ensuring we can all benefit through space inspired healthcare technologies such as these.”

- Adaptix, the company that developed the cutting-edge machine, was nurtured at ESA’s business incubation center in Harwell, UK.

- Nick Appleyard, head of Business Applications at the European Space Agency said: “This is a wonderful example of how ESA supports innovation. Adaptix started life in ESA’s UK Business Incubation Center and has grown to become a successful and innovative enterprise.”

- Mark Evans, chief executive of Adaptix Limited, said: “Working with ESA’s business incubation center hosted by the Rutherford Appleton Laboratory in Harwell has given us access to fantastic facilities and leading minds. ESA’s focus on commercializing space-heritage technology to create tangible benefits for the EU population and the UK economy has helped us to create 33 high-value UK jobs in research and development and, increasingly, in manufacturing.

- “Our vision is to create a business that will transform radiology through the export of high-science-content high-value products to achieve revenues of more than $100 m. X-ray is the primary diagnostic in healthcare and one day we hope that Adaptix technology will touch the life of everyone that you know.”

- The €1.2 m grant is due to come from an innovation fund drawn from ESA’s Business Applications and Space Solutions program, supported by the UK Space Agency.

• March 20, 2019: By surveying the center of our Galaxy, ESA’s XMM-Newton has discovered two colossal ‘chimneys’ funneling material from the vicinity of the Milky Way’s supermassive black hole into two huge cosmic bubbles. 33) 34)

- The giant bubbles were discovered in 2010 by NASA’s Fermi Gamma-ray Space Telescope: one stretches above the plane of the Milky Way galaxy and the other below, forming a shape akin to a colossal hourglass that spans about 50,000 light years – around half the diameter of the entire Galaxy. They can be thought of as giant ‘burps’ of material from the central regions of our Milky Way, where its central black hole, known as Sagittarius A*, resides.

- Now, XMM-Newton has discovered two channels of hot, X-ray emitting material streaming outwards from Sagittarius A*, finally linking the immediate surroundings of the black hole and the bubbles together.

- “We know that outflows and winds of material and energy emanating from a galaxy are crucial in sculpting and altering that galaxy’s shape over time – they are key players in how galaxies and other structures form and evolve throughout the cosmos,” says lead author Gabriele Ponti of the MPE (Max Planck Institute for Extraterrestrial Physics) in Garching, Germany, and the National Institute for Astrophysics in Italy.

- “Luckily, our Galaxy gives us a nearby laboratory to explore this in detail, and probe how material flows out into the space around us. We used data gathered by XMM-Newton between 2016 and 2018 to form the most extensive X-ray map ever made of the Milky Way’s core.”

- This map (Figure 33) revealed long channels of super-heated gas, each extending for hundreds of light years, streaming above and below the plane of the Milky Way.

- Scientists think that these act as a set of exhaust pipes through which energy and mass are transported from our Galaxy’s heart out to the base of the bubbles, replenishing them with new material.

- This finding clarifies how the activity occurring at the core of our home Galaxy, both present and past, is connected to the existence of larger structures around it.

- The outflow might be a remnant from our Galaxy’s past, from a period when activity was far more prevalent and powerful, or it may prove that even ‘quiescent’ galaxies – those that host a relatively quiet supermassive black hole and moderate levels of star formation like the Milky Way – can boast huge, energetic outflows of material.

- “The Milky Way is seen as a kind of prototype for a standard spiral galaxy,” says co-author Mark Morris of the University of California, Los Angeles, USA.

- “In a sense, this finding sheds light on how all typical spiral galaxies – and their contents – may behave across the cosmos.”

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Figure 33: An X-ray view of the center of our Milky Way galaxy, where the supermassive black hole Sagittarius A* is hosted. This image, obtained with ESA’s XMM-Newton space observatory, shows the temperature of the X-ray emitting gas in this turbulent region, with cooler regions shown in red and hotter regions in green and blue. The bright area at the middle of the image identifies the vicinity of Sagittarius A*. The yellow-orange features streaming above and below the center are two colossal ‘chimneys’, extending hundreds of light-years each, that funnel material from the Galactic center into two huge cosmic bubbles. This view combines data collected in the following energy bands: 1.5–2.6 keV (shown in red); 2.35– 2.56 keV (shown in green); 2.7–2.97 keV band (shown in blue). The many white patches, large and small, are artifacts where unrelated, bright, point-like X-ray sources have been removed from the image (image credit: ESA/XMM-Newton/G. Ponti et al. 2019, Nature)

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Figure 34: Artist’s impression of two ‘chimneys’ funneling hot, X-ray emitting material from the center of our Galaxy into two huge cosmic bubbles. The two galactic chimneys were revealed using data collected between 2016 and 2018 by ESA’s XMM-Newton space observatory, which completed the most extensive X-ray map ever made of the Milky Way’s core. The giant, gamma-ray emitting bubbles had been discovered by NASA’s Fermi Gamma-ray Space Telescope. They form a shape akin to a colossal hourglass, spanning about 50 000 light years from end to end – comparable to the size of the Milky Way’s stellar disc, and to around half the diameter of the entire Galaxy (image credit: ESA/XMM-Newton/G. Ponti et al. 2019; ESA/Gaia/DPAC (Milky Way map), CC BY-SA 3.0 IGO)

Legend to Figure 34: The two hot channels found by XMM-Newton stream outwards from Sagittarius A*, our Galaxy’s central supermassive black hole, and extend each for hundreds of light years, finally linking the immediate surroundings of the black hole and the bubbles together. Scientists think that these ‘chimneys’ act as a set of exhaust pipes through which energy and mass are transported from our Galaxy’s heart out to the base of the bubbles, replenishing them with new material.

- Despite its categorization as quiescent on the cosmic scale of galactic activity, previous data from XMM-Newton have revealed that our Galaxy’s core is still quite tumultuous and chaotic. Dying stars explode violently, throwing their material out into space; binary stars whirl around one another; and Sagittarius A*, a black hole as massive as four million Suns, lies in wait for incoming material to devour, later belching out radiation and energetic particles as it does so.

- Cosmic behemoths such as Sagittarius A* – and those even more massive – hosted by galaxies across the cosmos will be explored in depth by upcoming X-ray observatories like ESA’s Athena, the Advanced Telescope for High-Energy Astrophysics, scheduled for launch in 2031. Another future ESA mission, Lisa, the Laser Interferometer Space Antenna, will search for gravitational waves released by the merger of supermassive black holes at the core of distant, merging galaxies.

- Meanwhile, scientists are busy investigating these black holes with current missions like XMM-Newton.

- “There’s still a great deal to be done with XMM-Newton – the telescope could scan a significantly larger region of the Milky Way’s core, which would help us to map the bubbles and hot gas surrounding our Galaxy as well as their connections to the other components of the Milky Way, and hopefully figure out how all of this is linked together,” adds Gabriele.

- “Of course, we’re also looking forward to Athena and the breakthrough it will enable.”

- Athena will combine extremely high-resolution X-ray spectroscopy with excellent imaging capabilities over wide areas of the sky, allowing scientists to probe the nature and movement of hot cosmic gas like never before.

- “This outstanding result from XMM-Newton gives us an unprecedented view of what’s really happening at the core of the Milky Way, and presents the most extensive X-ray map ever created of the entire central region,” says ESA XMM-Newton Project Scientist Norbert Schartel.

- “This is especially exciting in the context of our upcoming missions. XMM-Newton is paving the way for the future generation of X-ray observatories, opening up abundant opportunities for these powerful spacecraft to make substantial new discoveries about our Universe.”

• February 25, 2019: Located some six million light-years away, the NGC 300 galaxy is relatively nearby. It is one of the closest galaxies beyond the Local Group – the hub of galaxies to which our own Milky Way galaxy belongs. Due to its proximity, it is a favorite target for astronomers to study stellar processes in spiral galaxies. 35)

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Figure 35: This swirling palette of colors portrays the life cycle of stars in a spiral galaxy known as NGC 300 (image credit: ESA/XMM-Newton (X-rays); MPG/ESO (optical); NASA/Spitzer (infrared). Acknowledgement: S. Carpano, Max-Planck Institute for Extraterrestrial Physics)

- The population of stars in their prime is shown in this image in green hues, based on optical observations performed with the Wide Field Imager (WFI) on the MPG/ESO 2.2-meter telescope at La Silla, Chile. Red colors indicate the glow of cosmic dust in the interstellar medium that pervades the galaxy: this information derives from infrared observations made with NASA’s Spitzer space telescope, and can be used to trace stellar nurseries and future stellar generations across NGC 300.

- A complementary perspective on this galaxy’s composition comes from data collected in X-rays by ESA’s XMM-Newton space observatory, shown in blue. These represent the end points of the stellar life cycle, including massive stars on the verge of blasting out as supernovas, remnants of supernova explosions, neutron stars, and black holes. Many of these X-ray sources are located in NGC 300, while others – especially towards the edges of the image – are foreground objects in our own Galaxy, or background galaxies even farther away.

- The sizeable blue blob immediately to the left of the galaxy’s center is especially interesting, featuring two intriguing sources that are part of NGC 300 and shine brightly in X-rays.

- One of them, known as NGC 300 X-1, is in fact a binary system, consisting of a Wolf-Rayet star – an ageing hot, massive and luminous type star that drives strong winds into its surroundings – and a black hole, the compact remains of what was once another massive, hot star. As matter from the star flows towards the black hole, it is heated up to temperatures of millions of degrees or more, causing it to shine in X-rays.

- The other source, dubbed NGC 300 ULX1, was originally identified as a supernova explosion in 2010. However, later observations prompted astronomers to reconsider this interpretation, indicating that this source also conceals a binary system comprising a very massive star and a compact object – a neutron star or a black hole – feeding on material from its stellar companion.

- Data obtained in 2016 with ESA’s XMM-Newton and NASA’s NuSTAR observatories revealed regular variations in the X-ray signal of NGC 300 ULX1, suggesting that the compact object in this binary system is a highly magnetized, rapidly spinning neutron star, or pulsar.

- The large blue blob in the upper left corner is a much more distant object: a cluster of galaxies more than one billion light years away, whose X-ray glow is caused by the hot diffuse gas interspersed between the galaxies.

• January 28, 2019: Investigating the history of our cosmos with a large sample of distant ‘active’ galaxies observed by ESA’s XMM-Newton, a team of astronomers found there might be more to the early expansion of the Universe than predicted by the standard model of cosmology. 36) 37)

- According to the leading scenario, our Universe contains only a few percent of ordinary matter. One quarter of the cosmos is made of the elusive dark matter, which we can feel gravitationally but not observe, and the rest consists of the even more mysterious dark energy that is driving the current acceleration of the Universe’s expansion.

- This model is based on a multitude of data collected over the last couple of decades, from the cosmic microwave background, or CMB – the first light in the history of the cosmos, released only 380 000 years after the big bang and observed in unprecedented detail by ESA’s Planck mission – to more ‘local’ observations. The latter include supernova explosions, galaxy clusters and the gravitational distortion imprinted by dark matter on distant galaxies, and can be used to trace cosmic expansion in recent epochs of cosmic history – across the past nine billion years.

- A new study, led by Guido Risaliti of Università di Firenze, Italy, and Elisabeta Lusso of Durham University, UK, points to another type of cosmic tracer – quasars – that would fill part of the gap between these observations, measuring the expansion of the Universe up to 12 billion years ago.

- Quasars are the cores of galaxies where an active supermassive black hole is pulling in matter from its surroundings at very intense rates, shining brightly across the electromagnetic spectrum. As material falls onto the black hole, it forms a swirling disc that radiates in visible and ultraviolet light; this light, in turn, heats up nearby electrons, generating X-rays.

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Figure 36: Supermassive black hole: Artist’s impression of a quasar, the core of a galaxy where an active supermassive black hole is pulling in matter from its surroundings at very intense rates. As material falls onto the black hole, it forms a swirling disc that radiates in visible and ultraviolet light; this light, in turn, heats up nearby electrons, generating X-rays. The relation between the ultraviolet and X-ray brightness of quasars can be used to estimate the distance to these sources – something that is notoriously tricky in astronomy – and, ultimately, to probe the expansion history of the Universe. -A team of astronomers has applied this method to a large sample of quasars observed by ESA’s XMM-Newton to investigate the history of our cosmos up to 12 billion years ago, finding there might be more to the early expansion of the Universe than predicted by the standard model of cosmology (image credit: ESA–C. Carreau)

- Three years ago, Guido and Elisabeta realized that a well-known relation between the ultraviolet and X-ray brightness of quasars could be used to estimate the distance to these sources – something that is notoriously tricky in astronomy – and, ultimately, to probe the expansion history of the Universe.

- Astronomical sources whose properties allow us to gauge their distances are referred to as ‘standard candles’.

- The most notable class, known as ‘type-Ia’ supernova, consists of the spectacular demise of white dwarf stars after they have over-filled on material from a companion star, generating explosions of predictable brightness that allows astronomers to pinpoint the distance. Observations of these supernovas in the late 1990s revealed the Universe’s accelerated expansion over the last few billion years.

- “Using quasars as standard candles has great potential, since we can observe them out to much greater distances from us than type-Ia supernovas, and so use them to probe much earlier epochs in the history of the cosmos,” explains Elisabeta.

- With a sizeable sample of quasars at hand, the astronomers have now put their method into practice, and the results are intriguing.

- Digging into the XMM-Newton archive, they collected X-ray data for over 7000 quasars, combining them with ultraviolet observations from the ground-based Sloan Digital Sky Survey. They also used a new set of data, specially obtained with XMM-Newton in 2017 to look at very distant quasars, observing them as they were when the Universe was only about two billion years old. Finally, they complemented the data with a small number of even more distant quasars and with some relatively nearby ones, observed with NASA’s Chandra and Swift X-ray observatories, respectively.

- “Such a large sample enabled us to scrutinize the relation between X-ray and ultraviolet emission of quasars in painstaking detail, which greatly refined our technique to estimate their distance,” says Guido.

- The new XMM-Newton observations of distant quasars are so good that the team even identified two different groups: 70 percent of the sources shine brightly in low-energy X-rays, while the remaining 30 percent emit lower amounts of X-rays that are characterized by higher energies. For the further analysis, they only kept the earlier group of sources, in which the relation between X-ray and ultraviolet emission appears clearer.

- “It is quite remarkable that we can discern such level of detail in sources so distant from us that their light has been travelling for more than ten billion years before reaching us,” says Norbert Schartel, XMM-Newton project scientist at ESA.

- After skimming through the data and bringing the sample down to about 1600 quasars, the astronomers were left with the very best observations, leading to robust estimates of the distance to these sources that they could use to investigate the Universe’s expansion.

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Figure 37: The graph showing measurements of the distance to astronomical objects such as type-Ia supernovas (cyan symbols) and quasars (yellow, red and blue symbols) that can be used to study the expansion history of the Universe. Type-Ia supernovas are the most notable class of ‘standard candles’– astronomical sources whose properties allow us to gauge their distances. They consist of the spectacular demise of white dwarf stars after they have over-filled on material from a companion star, generating explosions of predictable brightness that allows astronomers to pinpoint the distance. Observations of these supernovas in the late 1990s revealed the Universe’s accelerated expansion over the last few billion years [image credit: Elisabeta Lusso & Guido Risaliti (2019)]

- “When we combine the quasar sample, which spans almost 12 billion years of cosmic history, with the more local sample of type-Ia supernovas, covering only the past eight billion years or so, we find similar results in the overlapping epochs,” says Elisabeta. “However, in the earlier phases that we can only probe with quasars, we find a discrepancy between the observed evolution of the Universe and what we would predict based on the standard cosmological model.”

- Looking into this previously poorly explored period of cosmic history with the help of quasars, the astronomers have revealed a possible tension in the standard model of cosmology, which might require the addition of extra parameters to reconcile the data with theory.

- “One of the possible solutions would be to invoke an evolving dark energy, with a density that increases as time goes by,” says Guido.

- Incidentally, this particular model would also alleviate another tension that has kept cosmologists busy lately, concerning the Hubble constant – the current rate of cosmic expansion. This discrepancy was found between estimates of the Hubble constant in the local Universe, based on supernova data – and, independently, on galaxy clusters – and those based on Planck’s observations of the cosmic microwave background in the early Universe.

- “This model is quite interesting because it might solve two puzzles at once, but the jury is definitely not out yet and we’ll have to look at many more models in great detail before we can solve this cosmic conundrum,” adds Guido.

- The team is looking forward to observing even more quasars in the future to further refine their results. Additional clues will also come from ESA’s Euclid mission, scheduled for a 2022 launch to explore the past ten billion years of cosmic expansion and investigate the nature of dark energy.

- “These are interesting times to investigate the history of our Universe, and it’s exciting that XMM-Newton can contribute by looking at a cosmic epoch that had remained largely unexplored so far,” concludes Norbert.

• January 9, 2019: Astronomers using ESA's XMM-Newton space observatory have studied a black hole devouring a star and discovered an exceptionally bright and stable signal that allowed them to determine the black hole’s spin rate. 38)

- Black holes are thought to lurk at the center of all massive galaxies throughout the Universe, and are inextricably tied to the properties of their host galaxies. As such, revealing more about these behemoths may hold the key to understanding how galaxies evolve over time.

- A black hole’s gravity is extreme, and can rip apart stars that stray too close. The debris from such torn-apart stars spirals inwards towards the hole, heats up, and emits intense X-rays.

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Figure 38: The cosmic source called ASASSN-14li, concealing a black hole at least one million times as massive as the Sun that shredded and devoured a nearby star, as viewed by the European Photon Imaging Camera (EPIC) on ESA's XMM-Newton X-ray observatory. Observations of ASASSN-14li have revealed an exceptionally bright and stable signal that oscillated over a period of 131 seconds for a long time: 450 days. By combining this with information about the black hole’s mass and size, the astronomers found that the hole must be spinning rapidly – at more than 50% of the speed of light – and that the signal came from its innermost regions (image credit: ESA/XMM-Newton)

- Despite the number of black holes thought to exist in the cosmos, many are dormant – there is no in-falling material to emit detectable radiation – and thus difficult to study. However, every few hundred thousand years or so, a star is predicted to pass near enough to a given black hole that it is torn apart. This offers a brief window of opportunity to measure some fundamental properties of the hole itself, such as its mass and the rate at which it is spinning.

- “It’s very difficult to constrain the spin of a black hole, as spin effects only emerge very close to the hole itself, where gravity is intensely strong and it’s difficult to see clearly,” says Dheeraj Pasham of the MIT Kavli Institute for Astrophysics and Space Research in Massachusetts, USA, and lead author of the new study. 39)

- “However, models show that the mass from a shredded star settles into a kind of inner disc that throws off X-rays. We guessed that finding instances where this disc glows especially brightly would be a good way to constrain a black hole's spin, but observations of such events weren’t sensitive enough to explore this region of strong gravity in detail – until now.”

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Figure 39: This artist's impression shows hot gas orbiting in a disc around a rapidly-spinning black hole. The elongated spot depicts an X-ray-bright region in the disc, which allows the spin of the black hole to be estimated. Studying the black hole devouring a star known as ASASSN-14li with ESA's XMM-Newton space observatory and NASA’s Chandra and Swift X-ray observatories, a team of astronomers has discovered an exceptionally bright and stable signal that allowed them to determine the black hole’s spin rate (image credit: NASA/CXC/M. Weiss)

- Dheeraj and colleagues studied an event called ASASSN-14li. ASASSN-14li was discovered by the ground-based All-Sky Automated Survey for SuperNovae (ASASSN) on 22 November 2014. The black hole tied to the event is at least one million times as massive as the Sun. “ASASSN-14li is nicknamed the ‘Rosetta Stone’ of these events,” adds Dheeraj. “All of its properties are characteristic of this type of event, and it has been studied by all currently operational major X-ray telescopes.”

- Using observations of ASASSN-14li from ESA’s XMM-Newton and NASA’s Chandra and Swift X-ray observatories, the scientists hunted for a signal that was both stable and showed a characteristic wave pattern often triggered when a black hole receives a sudden influx of mass – such as when devouring a passing star.

- “It’s an exceptional finding: such a bright signal that is stable for so long has never been seen before in the vicinity of any black hole,” adds co-author Alessia Franchini of the University of Milan, Italy. - “What’s more, the signal is coming from right near the black hole’s event horizon – beyond this point we can’t observe a thing, as gravity is so strong that even light can’t escape.”

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Figure 40: The host galaxy of ASASSN-14li, a black hole devouring a star, as observed by the NASA/ESA Hubble Space Telescope in optical wavelengths. The insert in the lower left shows the X-ray view obtained by NASA’s Chandra observatory. Observations of ASASSN-14li have revealed an exceptionally bright and stable signal that oscillated over a period of 131 seconds for a long time: 450 days. By combining this with information about the black hole’s mass and size, the astronomers found that the hole must be spinning rapidly – at more than 50% of the speed of light – and that the signal came from its innermost regions (image credit: X-ray: NASA/CXC/MIT/D. Pasham et al; Optical: HST/STScI/I. Arcavi)

- The study demonstrates a novel way to measure the spins of massive black holes: by observing their activity when they disrupt passing stars with their gravity. Such events may also help us to understand aspects of general relativity theory; while this has been explored extensively in ‘normal’ gravity, it is not yet fully understood in regions where gravity is exceptionally strong.

- “XMM-Newton is incredibly sensitive to these signals, more so than any other X-ray telescope,” says ESA’s XMM-Newton Project Scientist Norbert Schartel. “The satellite provides the long, uninterrupted, detailed exposures that are crucial to detecting signals such as these. “We’re only just beginning to understand the complex physics at play here. By finding instances where the mass from a shredded star glows especially brightly we can build a census of the black holes in the Universe, and probe how matter behaves in some of the most extreme areas and conditions in the cosmos.”

• November 21, 2018: Based on a new theoretical model, a team of scientists explored the rich data archive of ESA's XMM-Newton and NASA's Chandra space observatories to find pulsating X-ray emission from three sources. The discovery, relying on previous gamma-ray observations of the pulsars, provides a novel tool to investigate the mysterious mechanisms of pulsar emission, which will be important to understand these fascinating objects and use them for space navigation in the future. 40)

- Lighthouses of the Universe, pulsars are fast-rotating neutron stars that emit beams of radiation. As pulsars rotate and the beams alternatively point towards and away from Earth, the source oscillates between brighter and dimmer states, resulting in a signal that appears to 'pulse' every few milliseconds to seconds, with a regularity rivalling even atomic clocks.

- Pulsars are the incredibly dense, extremely magnetic, relics of massive stars, and are amongst the most extreme objects in the Universe. Understanding how particles behave in such a strong magnetic field is fundamental to understanding how matter and magnetic fields interact more generally.

- Originally detected through their radio emission, pulsars are now known to also emit other types of radiation, though typically in smaller amounts. Some of this emission is standard thermal radiation – the type that everything with a temperature above absolute zero emits. Pulsars release thermal radiation when they accrete matter, for example from another star.

- But pulsars also emit non-thermal radiation, as is often produced in the most extreme cosmic environments. In pulsars, non-thermal radiation can be created via two processes: synchrotron emission and curvature emission. Both processes involve charged particles being accelerated along magnetic field lines, causing them to radiate light that can vary in wavelength from radio waves to gamma-rays.

Figure 41: XMM-Newton's view of pulsar J1826-1256 [image credit: ESA/XMM-Newton/J. Li, DESY (Deutsches Elektronen Synchrotron), Germany]

- Non-thermal X-rays result mostly from synchrotron emission, while gamma-rays may come from so-called synchro-curvature emission – a combination of the two mechanisms. It is relatively easy to find pulsars that radiate gamma-rays – NASA's Fermi Gamma-Ray Space Telescope has detected more than 200 of them over the past decade, thanks to its ability to scan the whole sky. But only around 20 have been found to pulse in non-thermal X-rays.

- "Unlike gamma-ray detecting survey instruments, X-ray telescopes must be told exactly where to point, so we need to provide them with some sort of guidance," says Diego Torres, from the Institute of Space Sciences in Barcelona, Spain.

- Aware that there should be many pulsars emitting previously undetected non-thermal X-rays, Torres developed a model that combined synchrotron and curvature radiation to predict whether pulsars detected in gamma-rays could also be expected to appear in X-rays.

- "Scientific models describe phenomena that can't be experienced directly," explains Torres. -"This model in particular helps explain the emission processes in pulsars and can be used to predict the X-ray emission that we should observe, based on the known gamma-ray emission."

- The model describes the gamma-ray emission of pulsars detected by Fermi – specifically, the brightness observed at different wavelengths – and combines this information with three parameters that determine the pulsar emission. This allows a prediction of their brightness at other wavelengths, for instance in X-rays.

- Torres partnered with a team of scientists, led by Jian Li from the Deutsches Elektronen Synchrotron in Zeuthen near Berlin, Germany, to select three known gamma-ray emitting pulsars that they expected, based on the model, to also shine brightly in X-rays. They dug into the data archives of ESA's XMM-Newton and NASA's Chandra X-ray observatories to search for evidence of non-thermal X-ray emission from each of them.

- "Not only did we detect X-ray pulsations from all three of the pulsars, but we also found that the spectrum of X-rays was almost the same as predicted by the model," explains Li. — "This means that the model very accurately describes the emission processes within a pulsar."

- In particular, XMM-Newton data showed clear X-ray emission from PSR J1826-1256 – a radio quiet gamma-ray pulsar with a period of 110.2 milliseconds. The spectrum of light received from this pulsar was very close to that predicted by the model. X-ray emission from the other two pulsars, which both rotate slightly more quickly, was revealed using Chandra data.

- This discovery already represents a significant increase in the total number of pulsars known to emit non-thermal X-rays. The team expects that many more will be discovered over the next few years as the model can be used to work out where exactly to look for them.

- Finding more X-ray pulsars is important for revealing their global properties, including population characteristics. A better understanding of pulsars is also essential for potentially taking advantage of their accurate timing signals for future space navigation endeavors.

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Figure 42: Observed X-ray and gamma-ray emission from three pulsars: J1747-2958 (left), J2021+3651 (center), and J1826-1256 (right), image credit: Adapted from J. Li et al. (2018)

- The result is a step towards understanding the relationships between the emission by pulsars in different parts of the electromagnetic spectrum, enabling a robust way to predict the brightness of a pulsar at any given wavelength. This will help us better comprehend the interaction between particles and magnetic fields in pulsars and beyond.

- "This model can make accurate predictions of pulsar X-ray emission, and it can also predict the emission at other wavelengths, for example visible and ultraviolet," Torres continues. - "In the future, we hope to find new pulsars leading to a better understanding of their global properties."

- The study highlights the benefits of XMM-Newton's vast data archive to make new discoveries and showcases the impressive abilities of the mission to detect relatively dim sources. The team is also looking forward to using the next generation of X-ray space telescopes, including ESA's future Athena mission, to find even more pulsars emitting non-thermal X-rays. 41)

- "As the flagship of European X-ray astronomy, XMM-Newton is detecting more X-ray sources than any previous satellite. It is amazing to see that it is helping to solve so many cosmic mysteries," concludes Norbert Schartel, XMM-Newton Project Scientist at ESA.

November 14, 2018: The SPC (Science Program Committee) of ESA has confirmed the continued operations of ten scientific missions in the Agency's fleet up to 2022. After a comprehensive review of their scientific merits and technical status, the SPC has decided to extend the operation of the five missions led by ESA's Science Program: Cluster, Gaia, INTEGRAL, Mars Express, and XMM-Newton. The SPC also confirmed the Agency's contributions to the extended operations of Hinode, Hubble, IRIS, SOHO, and ExoMars TGO. 42)

- This includes the confirmation of operations for the 2019–2020 cycle for missions that had been given indicative extensions as part of the previous extension process, and indicative extensions for an additional two years, up to 2022.
Note: Every two years, all missions whose approved operations end within the following four years are subject to review by the advisory structure of the Science Directorate. Extensions are granted to missions that satisfy the established criteria for operational status and science return, subject to the level of financial resources available in the science program. These extensions are valid for the following four years, subject to a mid-term review and confirmation after two years.

- The decision was taken during the SPC meeting at ESA/ESAC (European Space Astronomy Center) near Madrid, Spain, on 14 November.

- ESA's science missions have unique capabilities and are prolific in their scientific output. Cluster, for example, is the only mission that, by varying the separation between its four spacecraft, allows multipoint measurements of the magnetosphere in different regions and at different scales, while Gaia is performing the most precise astrometric survey ever realized, enabling unprecedented studies of the distribution and motions of stars in the Milky Way and beyond.

- Many of the science missions are proving to be of great value to pursue investigations that were not foreseen at the time of their launch. Examples include the role of INTEGRAL and XMM-Newton in the follow-up of recent gravitational wave detections, paving the way for the future of multi-messenger astronomy, and the many discoveries of diverse exoplanets by Hubble.

- Collaboration between missions, including those led by partner agencies, is also of great importance. The interplay between solar missions like Hinode, IRIS and SOHO provides an extensive suite of complementary instruments to study our Sun; meanwhile, Mars Express and ExoMars TGO are at the forefront of the international fleet investigating the Red Planet.

- Another compelling factor to support the extension is the introduction of new modes of operation to accommodate the evolving needs of the scientific community, as well as new opportunities for scientists to get involved with the missions.

Table 3: Extended life for ESA's science missions 42)

• October 29, 2018: A gigantic cold front in the Perseus galaxy cluster has been observed by a trio of X-ray telescopes. The ancient cold front can be seen at the left of the image, drifting away from the much inner, younger front closer to the center. Galactic cold fronts are nothing like the cold fronts we experience on Earth – instead they are caused by galaxy clusters colliding into one another. The gravitational pull of a larger cluster tugs a smaller cluster closer, resulting in gas in the core of the cluster being sloshed around like liquid in a glass. This creates a cold front in a spiral pattern moving outwards from the core and these sloshing cold fronts can provide a probe of the intercluster medium. 43) 44) 45)

- Cold fronts are the oldest coherent structures in cool core clusters and this one has been moving away from the center of the cluster for over five billion years – longer than our Solar System has been in existence. The long curving structure spans around two million light years and is travelling at around 50 km/s.

- The Perseus galaxy cluster contains thousands of galaxies and a supermassive black hole at the center. The black hole is responsible for creating a harsh environment of sound waves and turbulence that should erode a cold front over time, smoothing out the previously sharp edges and creating gradual changes in density and temperature. Instead, the high-resolution Chandra image showed a surprisingly sharp edge on the cold front, and a temperature map revealed that the upper left of the cold front is split in two.

- The sharpness of the cold front suggests it has been preserved by strong magnetic fields wrapped around it, essentially acting as a shield against the harsh environment. This magnetic "draping" prevents the cold front from diffusing and is what has allowed it to survive so well for over five billion years as it drifts away from the center of the cluster.

- Aurora Simionescu and collaborators originally discovered the Perseus cold front in 2012 using data from ROSAT (the ROentgen SATellite), ESA's XMM-Newton Observatory, and Japan's Suzaku X-ray satellite. Chandra’s high-resolution X-ray vision allowed this more detailed work on the cold front to be performed.

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Figure 43: The image combines data from NASA's Chandra X-Ray observatory, ESA's XMM-Newton and the German Aerospace Center-led ROSAT satellite. Chandra also took a separate close-up of the upper left of the cold front, revealing some unexpected details (image credit: NASA/CXC/GSFC/S. Walker, ESA/XMM, ROSAT)

• October 8, 2018: Astronomers using ESA’s XMM-Newton space observatory have captured the X-ray glow (shown here in purple in Figure 44) emitted by the hot gas that pervades the galaxy cluster XLSSC006. 46)

- The cluster is home to a few hundreds of galaxies, large amounts of diffuse, X-ray bright gas, and even larger amounts of dark matter, with a total mass equivalent to some 500 trillion solar masses. Because of its distance from us, we are seeing this galaxy cluster as it was when the Universe was only about nine billion years old.

- The galaxies that belong to the cluster are concentrated towards the center, with two dominant members. Since galaxy clusters normally have only one major galaxy at their core, this suggests that XLSSC006 is undergoing a merger event.

- The X-ray data were obtained as part of the XXL Survey, XMM-Newton’s largest observational program to date, with follow-up observations performed by a number of other observatories around the world and in space. The latest XXL Survey release contains data for 365 galaxy clusters, tracing their large-scale distribution across cosmic history. These observations are helping astronomers refine our understanding of the Universe’s structure and evolution, and will serve as a reference for ESA’s future missions Euclid and Athena.

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Figure 44: Pictured in this view, where the X-ray data are combined with a three-color composite of optical and near-infrared data from the Canada-France-Hawaii Telescope, are a multitude of other galaxies. Some are closer to us than the cluster – like the spiral galaxy towards the top right – and some are farther away. The image also shows a handful of foreground stars belonging to our Milky Way galaxy, which stand out with their diffraction spikes (a common artefact of astronomical images), while the small purple dots sprinkled across the frame are point sources of X-rays, many of them beyond the Milky Way [image credit:ESA/XMM-Newton (X-rays); CFHT-LS (optical); XXL Survey]

• October 4, 2018: This mosaic shows the 365 galaxy clusters of the XXL Survey as imaged in X-rays by ESA's XMM-Newton space observatory. 47)

- The clusters are ordered by increasing distance from us, starting from the most nearby, at a redshift of 0.03, in the top left corner, all the way to the most distant one, at a redshift of 1.99 (the seventeenth cluster in the bottom row from the left); the last seven clusters in the bottom row have uncertain redshift.

- The XXL Survey is XMM-Newton's largest observational program to date. The second batch of data from the survey includes information on 365 galaxy clusters, which trace the large-scale structure of the Universe and its evolution through time, and on 26,000 active galactic nuclei (AGN).

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Figure 45: The 365 galaxy clusters of the XXL Survey – X-ray view (image credit: ESA/XMM-Newton/XXL Survey)

• September 20, 2018: A UK team of astronomers report the first detection of matter falling into a black hole at 30% of the speed of light, located in the center of the billion-light year distant galaxy PG211+143. The team, led by Professor Ken Pounds of the University of Leicester, used data from the European Space Agency's X-ray observatory XMM-Newton to observe the black hole. Their results appear in a new paper in Monthly Notices of the Royal Astronomical Society. 48) 49)

- Black holes are objects with such strong gravitational fields that not even light travels quickly enough to escape their grasp, hence the description 'black'. They are hugely important in astronomy because they offer the most efficient way of extracting energy from matter. As a direct result, gas in-fall — accretion — onto black holes must be powering the most energetic phenomena in the Universe.

- The center of almost every galaxy — like our own Milky Way — contains a so-called supermassive black hole, with masses of millions to billions of times the mass of our Sun. With sufficient matter falling into the hole, these can become extremely luminous, and are seen as a quasar or AGN (Active Galactic Nucleus).

- However black holes are so compact that gas is almost always rotating too much to fall in directly. Instead it orbits the hole, approaching gradually through an accretion disk — a sequence of circular orbits of decreasing size. As gas spirals inwards, it moves faster and faster and becomes hot and luminous, turning gravitational energy into the radiation that astronomers observe.

- The orbit of the gas around the black hole is often assumed to be aligned with the rotation of the black hole, but there is no compelling reason for this to be the case. In fact, the reason we have summer and winter is that the Earth's daily rotation does not line up with its yearly orbit around the Sun.

- Until now it has been unclear how misaligned rotation might affect the in-fall of gas. This is particularly relevant to the feeding of supermassive black holes since matter (interstellar gas clouds or even isolated stars) can fall in from any direction.

- Using data from XMM-Newton, Prof. Pounds and his collaborators looked at X-ray spectra (where X-rays are dispersed by wavelength) from the galaxy PG211+143. This object lies more than one billion light years away in the direction of the constellation Coma Berenices, and is a Seyfert galaxy, characterized by a very bright AGN resulting from the presence of the massive black hole at its nucleus.

- The researchers found the spectra to be strongly red-shifted, showing the observed matter to be falling into the black hole at the enormous speed of 30% of the speed of light, or around 100,000 km/s. The gas has almost no rotation around the hole, and is detected extremely close to it in astronomical terms, at a distance of only 20 times the hole's size (its event horizon, the boundary of the region where escape is no longer possible).

- The observation agrees closely with recent theoretical work, also at Leicester and using the UK's Dirac supercomputer facility simulating the 'tearing' of misaligned accretion disks. This work has shown that rings of gas can break off and collide with each other, cancelling out their rotation and leaving gas to fall directly towards the black hole.

- Prof. Pounds, from the University of Leicester's Department of Physics and Astronomy, said: "The galaxy we were observing with XMM-Newton has a 40 million solar mass black hole which is very bright and evidently well fed. Indeed some 15 years ago we detected a powerful wind indicating the hole was being over-fed. While such winds are now found in many active galaxies, PG1211+143 has now yielded another 'first', with the detection of matter plunging directly into the hole itself."

- He continues: "We were able to follow an Earth-sized clump of matter for about a day, as it was pulled towards the black hole, accelerating to a third of the velocity of light before being swallowed up by the hole."

- A further implication of the new research is that 'chaotic accretion' from misaligned disks is likely to be common for supermassive black holes. Such black holes would then spin quite slowly, being able to accept far more gas and grow their masses more rapidly than generally believed, providing an explanation for why black holes which formed in the early Universe quickly gained very large masses.

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Figure 46: This is the characteristic disk structure from the simulation of a misaligned disk around a spinning black hole. The outermost regions are warped and remain misaligned. Inside this, several rings have broken free and are freely precessing through the Lense–Thirring effect. The innermost material has fallen from the shocks that occur between rings, and is aligned to the central black hole spin (it is the misaligned component of angular momentum that is cancelled in the shocks – and transferred to the hole through precession). Depending on the observer’s line of sight, the infalling matter may or may not obscure the central emitting regions. Note that the black hole spin vector is drawn as an arrow up the page, but this is the projection on to the page. The black hole spin vector points out towards the reader as well as in the
direction drawn, such that the vector is normal to the innermost ring of gas (image credit: K. Pounds et al., University of Leicester)

• August 10, 2018: An enigmatic X-ray source revealed as part of a data-mining project for high-school students shows unexplored avenues hidden in the vast archive of ESA’s XMM-Newton X-ray Observatory. 50) 51)

Figure 47: Flaring source in NGC 6540: A peculiar X-ray source spotted in the globular cluster NGC 6540 as part of a collaboration between scientists at the National Institute of Astrophysics (INAF) in Milan, Italy, and a group of students from a local high school. In 2005, ESA’s XMM-Newton saw this source undergo a flare that boosted the luminosity of the source by up to 50 times its normal level for about five minutes. Too short to be an ordinary stellar flare, but too faint to be linked to a compact object, this event is challenging our understanding of X-ray outbursts [image credit: ESA/XMM-Newton, A. De Carlo (INAF)] 52)

- When XMM-Newton was launched in 1999, most students who are finishing high school today were not even born. Yet ESA's almost two-decade old X-ray observatory has many surprises to be explored by the next generation of scientists.

- A taste of new discoveries was unveiled in a recent collaboration between scientists at the National Institute of Astrophysics (INAF) in Milan, Italy, and a group of twelfth-grade students from a secondary school in nearby Saronno.

- The fruitful interaction was part of the Exploring the X-ray Transient and variable Sky project, EXTraS, an international research study of variable sources from the first 15 years of XMM-Newton observations.

- "We recently published the EXTraS catalog, which includes all the X-ray sources – about half a million – whose brightness changes over time as observed by XMM-Newton, and lists several observed parameters for each source," says Andrea De Luca, one of the scientists who coordinated the student project.

- "The next step was to delve into this vast data set and find potentially interesting sources, and we thought this would be an exciting challenge for a student internship," adds Andrea.

- Scientists at INAF in Milan have been cooperating with local schools for a few years, hosting several groups of students at the institute for a couple of weeks and embedding them in the activities of the various research groups.

- "For this particular project, the students received an introduction about astronomy and the exotic sources we study with X-ray telescopes, as well as a tutorial on the database and how to use it," explains Ruben Salvaterra, another scientist involved in the program. "Once they were ready to explore the data archive, they proved very effective and resourceful."

- The six students analyzed about 200 X-ray sources, looking at their light curve – a graph showing the object's variability over time – and checking the scientific literature to verify whether they had been studied already.

- Eventually, they identified a handful of sources exhibiting interesting properties – a powerful flare, for example – that had not been previously reported by other studies. "One of the sources stood out as especially intriguing," says Andrea.

- Featuring the shortest flare of all analyzed objects, this source appears to be located in the globular cluster NGC 6540 – a dense grouping of stars – and had not been studied before.

- After presenting their findings to the scientists in a seminar, the students went back to school. But the work for Andrea, Ruben and collaborators had only just begun.

- "The source identified by the students displays brightness changes like no other known objects, so we started looking more in detail," says Ruben.

- An otherwise low-luminosity source of X-rays, XMM-Newton saw it brighten by up to 50 times its normal level in 2005, and quickly fall again after about five minutes.

- Stars like our Sun shine moderately in X-rays, and occasionally undergo flares that boost their brightness like the one observed in this source. However, such events normally last much longer – up to a few hours or even days.

- On the other hand, short outbursts are observed in binary star systems hosting a dense stellar remnant such as neutron star, but these outpourings of X-rays are characterized by a much higher luminosity.

- "This event is challenging our understanding of X-ray outbursts: too short to be an ordinary stellar flare, but too faint to be linked to a compact object," explains collaborator Sandro Mereghetti, lead author of the paper presenting the results.

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Figure 48: These six students discovered a peculiar X-ray source in the archive of ESA's XMM-Newton during a two-week internship at INAF, Milan, in September 2017. Razvan Patrolea, Lorenzo Apollonio, Elena Pecchini, Cinzia Torrente, Bartolomeo Bottazzi-Baldi and Martino Giobbio from Liceo scientifico G.B. Grassi in Saronno, Italy (image credit: INAF)

- Another possibility is that the source is a so-called chromospherically active binary, a dual system of stars with intense X-ray activity caused by processes in their chromosphere, an intermediate layer in a star's atmosphere. But even in this case, it does not closely match the properties of any known object of this class.

- The scientists suspect that this peculiar source is not unique, and that other objects with similar properties are lurking in the XMM-Newton archive but have not yet been identified because of the combination of low luminosity and short duration of the flare.

- "The systematic study of variability that led to the compilation of the EXTraS catalog, together with this first attempt at data mining, suggests that we have opened a new, unexplored window on the X-ray Universe," adds Sandro. - The team plans to study the newly identified source in greater detail to better understand its nature, while searching for more similar objects in the archive.

- "It is exciting to find hidden jewels like this source in the XMM-Newton archive, and that young students are helping us find them while learning and having fun," concludes Norbert Schartel, XMM-Newton project scientist at ESA.

• July 25, 2018: Members of the X-ray astronomy working group at the Leibniz Institute for Astrophysics (AIP) in Potsdam and an international team have published the first catalog of X-ray sources in multiple observed sky regions. The catalog comprises almost 72,000 objects, partly of exotic nature, which were observed with the space-based X-ray telescope XMM-Newton. It provides information on the physical properties of the sources and enables astronomers to identify brightness variations on time scales of several years - and includes several thousand new detections. 53) 54)

- Since its launch end of 1999, the European X-ray satellite XMM-Newton has observed many patches of the sky repeatedly. Members of the X-ray astronomy group have developed new software to search for astrophysical objects in overlapping observations and used it to compile the first catalog. By combining multiple observations of the same region of sky, higher accuracy is reached and faint sources are found that are not detectable in the individual observations. "Our method is similar to combining several transparencies showing the same subject: The more images are superimposed the more details become visible," explains Dr. Iris Traulsen, the project scientist at the AIP.

Figure 49: Nineteen superimposed XMM-Newton observations of the same sky region. This corresponds to an exposure time of more than three days (image credit: AIP)

- The new catalog comprises 71,951 X-ray sources in 1,789 XMM-Newton observations and lists a wealth of information on their physical properties. Several thousand of these sources are newly discovered, many of them very faint and difficult to detect. The catalog can be used to trace brightness changes of X-ray sources over time scales of up to 14.5 years. Dr. Axel Schwope, team leader at the AIP, says: "Variations of the X-ray brightness are an essential criteria used to search for exotic Celestial objects. To decipher their nature, we also employ the LBT (Large Binocular Telescope) in Arizona." The AIP is one of the LBT partners and contributes to its instrumentation and software.

- Scientists all over the world have been using the XMM-Newton Source Catalogs to get new information about their research objects and to search for rare and as yet unknown sources of X-rays. 55)

• June 20, 2018: After a nearly twenty-year long game of cosmic hide-and-seek, astronomers using ESA's XMM-Newton space observatory have finally found evidence of hot, diffuse gas permeating the cosmos, closing a puzzling gap in the overall budget of 'normal' matter in the Universe. 56)

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Figure 50: Artist's impression of the warm-hot intergalactic medium, a mixture of gas with temperatures ranging from hundreds of thousands of degrees (warm) to millions of degrees (hot) that permeated the Universe in a filamentary cosmic web (image credit: Illustrations and composition: ESA / ATG medialab; data: ESA / XMM-Newton / F. Nicastro et al. 2018; cosmological simulation: R. Cen)

- While the mysterious dark matter and dark energy make up about 25 and 70 percent of our cosmos respectively, the ordinary matter that makes up everything we see – from stars and galaxies to planets and people – amounts to only about five percent. - But even this five percent turns out to be quite hard to track down.

- The total amount of ordinary matter, which astronomers refer to as baryons, can be estimated from observations of the Cosmic Microwave Background, which is the most ancient light in the history of the Universe, dating back to only about 380 000 years after the Big Bang.

- Observations of very distant galaxies allow astronomers to follow the evolution of this matter throughout the Universe's first couple billions of years. After that, however, more than half of it seemed to have gone missing.

- "The missing baryons represent one of the biggest mysteries in modern astrophysics," explains Fabrizio Nicastro, lead author of the paper presenting a solution to the mystery, published today in Nature. "We know this matter must be out there, we see it in the early Universe, but then we can no longer get hold of it. Where did it go?" 57)

- Counting the population of stars in galaxies across the Universe, plus the interstellar gas that permeates galaxies – the raw material to create stars – only gets as far as a mere ten percent of all ordinary matter. Adding up the hot, diffuse gas in the haloes that encompass galaxies and the even hotter gas that fills galaxy clusters, which are the largest cosmic structures held together by gravity, raises the inventory to less than twenty percent.

- This is not surprising: stars, galaxies and galaxy clusters form in the densest knots of the cosmic web, the filamentary distribution of both dark and ordinary matter that extends throughout the Universe. While these sites are dense, they are also rare, so not the best spots to look for the majority of cosmic matter.

- Astronomers suspected that the 'missing' baryons must be lurking in the ubiquitous filaments of this cosmic web, where matter is however less dense and therefore more challenging to observe. Using different techniques over the years, they were able to locate a good chunk of this intergalactic material – mainly its cool and warm components – bringing up the total budget to a respectable 60 percent, but leaving the overall mystery still unsolved.

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Figure 51: The cosmic budget of 'ordinary' matter (image credit: ESA)

- Fabrizio and many other astronomers around the world have been on the tracks of the remaining baryons for almost two decades, ever since X-ray observatories such as ESA's XMM-Newton and NASA's Chandra became available to the scientific community.

- Observing in this portion of the electromagnetic spectrum, they can detect hot intergalactic gas, with temperatures around a million degrees or more, that is blocking the X-rays emitted by even more distant sources.

- For this project, Fabrizio and his collaborators used XMM-Newton to look at a quasar – a massive galaxy with a supermassive black hole at its center that is actively devouring matter and shining brightly from X-rays to radio waves. They observed this quasar, whose light takes more than four billion years to reach us, for a total of 18 days, split between 2015 and 2017, in the longest X-ray observation ever performed of such a source.

- "After combing through the data, we succeeded at finding the signature of oxygen in the hot intergalactic gas between us and the distant quasar, at two different locations along the line of sight," says Fabrizio. "This is happening because there are huge reservoirs of material – including oxygen – lying there, and just in the amount we were expecting, so we finally can close the gap in the baryon budget of the Universe."

- This extraordinary result is the beginning of a new quest. Observations of different sources across the sky are needed to confirm whether these findings are truly universal, and to further investigate the physical state of this long-sought-for matter.

- Fabrizio and his colleagues are planning to study more quasars with XMM-Newton and Chandra in the coming years. To fully explore the distribution and properties of this so-called warm-hot intergalactic medium, however, more sensitive instruments will be needed, like ESA's Athena, the Advanced Telescope for High-Energy Astrophysics, scheduled for launch in 2028.

- "The discovery of the missing baryons with XMM-Newton is the exciting first step to fully characterize the circumstances and structures in which these baryons are found," says co-author Jelle Kaastra from the Netherlands Institute for Space Research. "For the next steps, we will need the much higher sensitivity of Athena, which has the study of the warm-hot intergalactic medium as one of its main goals, to improve our understanding of how structures grow in the history of the Universe."

- "It makes us very proud that XMM-Newton was able to discover the weak signal of this long elusive material, hidden in a million-degree hot fog that extends through intergalactic space for hundreds of thousands of light years," says Norbert Schartel, XMM-Newton project scientist at ESA. "Now that we know these baryons are no longer missing, we can't wait to study them in greater detail."

• June 18, 2018: ESA’s XMM-Newton observatory has discovered the best-ever candidate for a very rare and elusive type of cosmic phenomenon: a medium-weight black hole in the process of tearing apart and feasting on a nearby star. 58) 59)

- There are various types of black hole lurking throughout the Universe: massive stars create stellar-mass black holes when they die, while galaxies host supermassive black holes at their centers, with masses equivalent to millions or billions of Suns.

- Lying between these extremes is a more retiring member of the black hole family: intermediate-mass black holes. Thought to be seeds that will eventually grow to become supermassive, these black holes are especially elusive, and thus very few robust candidates have ever been found.

- Now, a team of researchers using data from ESA’s XMM-Newton X-ray space observatory, as well as NASA’s Chandra X-Ray Observatory and Swift X-Ray Telescope, has found a rare telltale sign of activity. They detected an enormous flare of radiation in the outskirts of a distant galaxy, thrown off as a star passed too close to a black hole and was subsequently devoured.

- “This is incredibly exciting: this type of black hole hasn’t been spotted so clearly before,” says lead scientist Dacheng Lin of the University of New Hampshire, USA. “A few candidates have been found, but on the whole they’re extremely rare and very sought after. This is the best intermediate-mass black hole candidate observed so far.”

- This breed of black hole is thought to form in various ways. One formation scenario is the runaway merger of massive stars lying within dense star clusters, making the centers of these clusters one of the best places to hunt for them. However, by the time such black holes have formed, these sites tend to be devoid of gas, leaving the black holes with no material to consume and thus little radiation to emit – which in turn makes them extremely difficult to spot.

- "One of the few methods we can use to try to find an intermediate-mass black hole is to wait for a star to pass close to it and become disrupted – this essentially 'activates' the black hole's appetite again and prompts it to emit a flare that we can observe," adds Lin. "This kind of event has only been clearly seen at the center of a galaxy before, not at the outer edges."

- Lin and colleagues sifted through data from XMM-Newton to find the candidate. They identified it in observations of a large galaxy some 740 million light-years away, taken in 2006 and 2009 as part of a galaxy survey, and in additional data from Chandra (2006 and 2016) and Swift (2014).

- "We also looked at images of the galaxy taken by a whole host of other telescopes, to see what the emission looked like optically," says co-author Jay Strader of Michigan State University, USA. "We spotted the source flaring in brightness in two images from 2005 – it appeared far bluer and brighter than it had just a few years previously. By comparing all the data we determined that the unfortunate star was likely disrupted in October 2003 in our time, and produced a burst of energy that decayed over the following 10 years or so."

- The scientists believe that the star was disrupted and torn apart by a black hole with a mass of around fifty thousand times that of the Sun.

- Such star-triggered outbursts are expected to only happen rarely from this type of black hole, so this discovery suggests that there could be many more lurking in a dormant state in galaxy peripheries across the local Universe.

- "This candidate was discovered via an intensive search of XMM-Newton's X-ray Source Catalog, which is filled with high-quality data covering large areas of sky, essential for determining how large the black hole was and what happened to cause the observed burst of radiation," says Norbert Schartel, ESA Project Scientist for XMM-Newton.

- "The XMM-Newton X-ray Source Catalogue is presently the largest catalogue of this type, containing more than half a million sources: exotic objects like the one discovered in our study are still hidden there and waiting to be discovered through intensive data mining," adds co-author Natalie Webb, director of the XMM-Newton Survey Science Center at the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France.

- "Learning more about these objects and associated phenomena is key to our understanding of black holes. Our models are currently akin to a scenario in which an alien civilization observes Earth and spots grandparents dropping their grandchildren at pre-school: they might assume that there's something intermediate to fit their model of a human lifespan, but without observing that link, there's no way to know for sure. This finding is incredibly important, and shows that the discovery method employed here is a good one to use," concludes Norbert.

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Figure 52: Best ever intermediate-mass black hole candidate (purple spot) at the outskirts of a distant galaxy (image credit: Optical: NASA/ESA/Hubble/STScI; X-ray: NASA/CXC/UNH/D. Lin et al.)

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Figure 53: .XMM-Newton view of intermediate-mass black hole candidate. The X-ray source 3XMM J215022.4-055108, viewed with ESA’s XMM-Newton X-ray space observatory in 2006 (left) and 2009 (right), image credit: ESA/XMM-Newton; D. Lin et al (University of New Hampshire, USA); Acknowledgement: NASA/CXC

• June 11, 2018: This turbulent celestial palette of purple and yellow shows a bubble of gas named NGC 3199 , blown by a star known as WR18 (Wolf-Rayet 18). 60)

- Wolf-Rayet stars are massive, powerful, and energetic stars that are just about reaching the end of their lives. They flood their surroundings with thick, intense, fast-moving winds that push and sweep at the material found there, carving out weird and wonderful shapes as they do so. These winds can create strong shockwaves when they collide with the comparatively cool interstellar medium, causing them to heat up anything in their vicinity. This process can heat material to such high temperatures that it is capable of emitting X-rays, a type of radiation emitted only by highly energetic phenomena in the Universe.

- This is what has happened in the case of NGC 3199. Although this kind of scenario has been seen before, it is still relatively rare; only three other Wolf-Rayet bubbles have been seen to emit X-rays (NGC 2359, NGC 6888, and S308). WR18 is thought to be a star with especially powerful winds; once it has run out of material to fuel these substantial winds it will explode violently as a supernova, creating a final breath-taking blast as it ends its stellar life.

- The image of Figure 54 was taken by the EPIC (European Photon Imaging Camera) on ESA’s XMM-Newton X-ray space observatory, and marks different patches of gas in different colors. The incredibly hot, diffuse, X-ray-emitting gas within the Wolf-Rayet bubble is shown in blue, while a bright arc that is visible in the optical part of the spectrum is traced out in shades of yellow-green (oxygen emission) and red (sulphur emission).

- The blue and yellow-green component forms an optical nebula – a glowing cloud of dust and ionized gases – that stretches out towards the western end of the X-ray bubble (in this image, North is to the upper left). This lopsided arc caused astronomers to previously identify WR18 as a so-called runaway star moving far faster than expected in relation to its surroundings, but more recent studies have shown that the observed X-ray emission does not support this idea. Instead, the shape of NGC 3199 is thought to be due to variations in the chemistry of the bubble’s surroundings, and the initial configuration of the interstellar medium around WR18.

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Figure 54: This star-circling bubble was spotted throwing out X-rays by ESA’s XMM-Newton, earning it entry to a select group of stellar objects (image credit: ESA/XMM-Newton; J. Toalá; D. Goldman)

• May 31, 2018: Last year, the first detection of gravitational waves linked to a gamma-ray burst triggered a vast follow-up campaign with ground and space telescopes to study the aftermath of the neutron star merger that gave rise to the explosion. ESA's XMM-Newton observations, obtained a few months after the discovery, caught the moment when its X-ray emission stopped increasing, opening new questions about the nature of this peculiar source. 61)

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Figure 55: Neutron star merger in galaxy NGC 4993. The elliptical galaxy NGC 4993, about 130 million light-years from Earth, viewed by ESA's XMM-Newton X-ray observatory. The bright region to the upper left of the galaxy corresponds to the remnant of the neutron star merger that was first detected as a gravitational wave source by the LIGO/Virgo collaboration (and as a gamma-ray burst by ESA's Integral and NASA's Fermi satellites) on 17 August 2017. XMM-Newton observed this source on 29 December 2017, obtaining the first evidence that its X-ray brightness, after four months of constant rise, had stopped increasing. The further evolution of this system will provide new insight into its geometry and the explosion that created it (image credit: ESA/XMM-Newton; P. D'Avanzo (INAF–Osservatorio Astronomico di Brera)

- Gravitational waves, predicted by Albert Einstein's general theory of relativity in 1918, are ripples in the fabric of spacetime caused by accelerating massive objects like colliding pairs of neutron stars or black holes.

- These fluctuations, which remained elusive for a century after the prediction, can now be detected using giant experiments on the ground such as the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and Europe's Virgo interferometer.

- After a gravitational wave detection, scientists mobilize a large number of ground-based and spaceborne astronomical facilities to look for a possible counterpart of the waves across the electromagnetic spectrum and learn more about their source.

- All but one of the six gravitational-wave events that have been observed since 2015 had no evidence of an electromagnetic counterpart, in agreement with the fact that they originated from the merger of black holes – a cosmic phenomenon that is not expected to release any light.

- This is why the first detection of gravitational waves jointly with gamma rays, on 17 August 2017, gave rise to a worldwide sensation, launching an observing campaign that involved observatories across the globe and in space to follow the evolution of this never-before-seen phenomenon.

- ESA's INTEGRAL and NASA's Fermi gamma-ray satellites had detected the blast only two seconds after its gravitational waves had passed through the LIGO and Virgo detectors, linking the gamma-ray burst to the source of the spacetime ripples, caused by the coalescence of two neutron stars – dense remnants that form at the end of a massive star's life.

- Scientists then looked for the afterglow of the explosion created by the neutron star merger, which they expected to observe at longer wavelengths, from X-rays to radio waves. While the optical signal was received about half a day after the original detection, it took no less than nine days for the first observations of this object in X-rays and radio waves.

- The delay of the X-ray and radio afterglow contains information about the geometry of the explosion, suggesting that it might have generated two symmetric and collimated jets, neither of which, however, pointed towards Earth.

- The X-ray observations were performed with NASA's Chandra X-ray Observatory and other space telescopes. Chandra kept an eye on this source during the following months, recording an ever increasing trend in its X-ray brightness.

- Due to observational constraints, XMM-Newton could not observe the aftermath of this cosmic clash for the first four months after its first detection. When it eventually did so, on 29 December 2017, the X-ray brightness seemed to have stopped rising.

- "The XMM-Newton observations had a very good timing," explains Paolo D'Avanzo from INAF – Osservatorio Astronomico di Brera, Italy. D'Avanzo is the lead author of the paper reporting the results, published this month in Astronomy & Astrophysics. 62)

- "By measuring the same value seen by Chandra earlier that month, XMM-Newton provided the first evidence that the source had reached its X-ray peak, and that its incessant brightening had finally come to a halt," he adds. "This was later confirmed by another team of scientists who keep monitoring the source with Chandra."

- Scientists expected that the X-ray brightness would reach a peak after a few months, as the material that had been ejected and heated up by the explosion slowly decelerated into the surrounding interstellar medium. The further evolution of the system, however, could still have some surprises in store.

- If the explosion did produce two symmetric jets that are not pointing towards Earth, as inferred from the first observations, its X-ray output will decrease rapidly.

- But there is another possibility that could explain the data obtained so far: the explosion could have also happened as a spherical 'fireball', without jets, but with a much lower energy. In this case, the X-ray brightness would decrease at a more leisurely pace after the peak.

- "We are eager to see how this source will behave over the coming months, since it will tell us whether we are looking off-axis at a beamed gamma-ray burst, as we thought until now, or witnessing a different phenomenon," says D'Avanzo. "This coincidentally well-timed observation is taking us one step closer to understanding the nature of this unique source," says Norbert Schartel, XMM-Newton Project Scientist at ESA.

- In what scientists call a multi-messenger approach, observations across the electromagnetic spectrum are key to study in-depth this and similar sources of gravitational waves that will be discovered in future years by LIGO and Virgo.

- The two gravitational wave experiments will start their observations again, with improved sensitivity, at the beginning of 2019, while ESA's future mission, LISA, the Laser Interferometer Space Antenna, which will observe lower frequency gravitational waves from space, is planned for launch in 2034.

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Figure 56: The elliptical galaxy NGC 4993, about 130 million light-years from Earth, viewed with the VIMOS instrument on the European Southern Observatory's Very Large Telescope in Chile. After the almost simultaneous detection of gravitational waves by the LIGO/Virgo collaboration and of a gamma-ray burst by ESA's INTEGRAL and NASA's Fermi satellites, a large number of ground and space telescopes started searching for the source in the sky (image credit: ESO/A. J. Levan, N.R. Tanvir, CC BY 4.0, Ref. 61)

• April 18, 2018: Astronomers using ESA’s XMM-Newton space observatory have probed the gas-filled haloes around galaxies in a quest to find ‘missing’ matter thought to reside there, but have come up empty-handed – so where is it? 63)

- All the matter in the Universe exists in the form of ‘normal’ matter or the notoriously elusive and invisible dark matter, with the latter around six times more prolific.

- Curiously, scientists studying nearby galaxies in recent years have found them to contain three times less normal matter than expected, with our own Milky Way Galaxy containing less than half the expected amount. “This has long been a mystery, and scientists have spent a lot of effort searching for this missing matter,” says Jiangtao Li of the University of Michigan, USA, and lead author of a new paper. 64) “Why is it not in galaxies — or is it there, but we are just not seeing it? If it’s not there, where is it? It is important we solve this puzzle, as it is one of the most uncertain parts of our models of both the early Universe and of how galaxies form.”

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Figure 57: This image illustrates the X-ray emission around a set of five galaxies that have been stacked together to bring out the details in their spherical, gaseous haloes. It was created by a team of scientists using ESA’s XMM-Newton space observatory, with the X-ray emission highlighted in purple [image credit: ESA/XMM-Newton; J-T. Li (University of Michigan, USA); Sloan Digital Sky Survey (SDSS)] 65)

- Rather than lying within the main bulk of the galaxy, the part can be observed optically, researchers thought it may instead lie within a region of hot gas that stretches further out into space to form a galaxy’s halo.

- These hot, spherical haloes have been detected before, but the region is so faint that it is difficult to observe in detail – its X-ray emission can become lost and indistinguishable from background radiation. Often, scientists observe a small distance into this region and extrapolate their findings but this can result in unclear and varying results.

- Jiangtao and colleagues wanted to measure the hot gas out to larger distances using ESA’s XMM-Newton X-ray space observatory. They looked at six similar spiral galaxies and combined the data to create one galaxy with their average properties.

- “By doing this, the galaxy’s signal becomes stronger and the X-ray background becomes better behaved,” adds co-author Joel Bregman, also of the University of Michigan. “We were then able to see the X-ray emission to about three times further out than if observing a single galaxy, which made our extrapolation more accurate and reliable.”

- Massive and isolated spiral galaxies offer the best chance to search for missing matter. They are massive enough to heat gas to temperatures of millions of degrees so that they emit X-rays, and have largely avoided being contaminated by other material through star formation or interactions with other galaxies.

Still missing:

- The team’s results showed that the halo surrounding galaxies like the ones observed cannot contain all of the missing matter after all. Despite extrapolating out to almost 30 times the radius of the Milky Way, nearly three-quarters of the expected material was still missing.

- There are two main alternative theories as to where it could be: either it is stored in another gas phase that is poorly observed – perhaps either a hotter and more tenuous phase or a cooler and denser one – or within a patch of space that is not covered by our current observations or emits X-rays too faintly to be detected.

- Either way, since the galaxies do not contain enough missing matter they may have ejected it out into space, perhaps driven by injections of energy from exploding stars or by supermassive black holes.

- “This work is important to help create more realistic galaxy models, and in turn help us better understand how our own Galaxy formed and evolved,” says Norbert Schartel, ESA XMM-Newton project scientist. “This kind of finding is simply not possible without the incredible sensitivity of XMM-Newton. - In the future, scientists can add even more galaxies to our study samples and use XMM-Newton in collaboration with other high-energy observatories, such as ESA’s upcoming ATHENA (Advanced Telescope for High-ENergy Astrophysics) mission, to probe the extended, low-density parts of a galaxy’s outer edges, as we continue to unravel the mystery of the Universe’s missing matter.”

• March 19, 2018: The Crab Nebula is a supernova remnant some 6500 light-years from Earth in the constellation of Taurus. At the center of the nebula is a pulsar – the remnant of a star that exploded to form the nebula. The pulsar rotates around 30 times a second, sweeping a beam of radio waves across the Galaxy. Some of the material surrounding the pulsar was ejected before the star exploded, and the rest was expelled during the supernova. The wind from the pulsar escapes at high speed, creating a dynamic structure by interacting with the ejected material. 66)

- The nebula is currently expanding at around 1500 km/s, as revealed by images taken a few years apart. By tracing this backwards it is possible to pinpoint the year in which the star exploded, and this coincides with observations by Chinese astronomers in 1054 of a star bright enough to be seen during daylight.

- The image shown in Figure 58 is in ultraviolet light taken by ESA’s XMM-Newton telescope, which has been surveying the sky since 2000. While this is primarily a telescope for observing X-rays, the Optical Monitor enables optical and ultraviolet observations to be made simultaneously with X-ray observations. The image is a composite of 75 individual images taken between 2001 and 2015. Very few ultraviolet images of the Crab Nebula were available before this one.

- The ultraviolet emission is thought to come from ‘synchrotron radiation’, created when atomic particles spiral around magnetic field lines. The XMM-Newton image reveals ‘bays’ indenting the east and west sides of the nebula. It is thought that a magnetized torus of material surrounded the star before it exploded, which then blocked the high-speed particles and thus the synchrotron radiation. The bays are also evident in radio images, although the eastern bay is better defined owing to intricate features around the borders of the radio image.

- A new composite of the Crab Nebula comprising NASA Chandra and Spitzer data and NASA/ESA Hubble data was also released last week.

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Figure 58: This image was taken as part of detailed multi-wavelength study of the Crab Nebula, with images also taken in X-rays, radio waves, infrared and optical wavelengths (image credit: ESA)