XMM-Newton (X-ray Multi-Mirror Mission-Newton) Observatory
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)
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).
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)
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.
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.
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.
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.
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:
- 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.
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.
Figure 10: Photo of the XMM telescope during wide-angle stray-light testing at Dornier (Ottobrunn, Germany), image credit: ESA
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.
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. 118).
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
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.
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.
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)
• 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. 16)
- 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 16: 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. 17)
- “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 17: 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’.
Figure 18: 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. 18)
Figure 19: 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. 19)
• 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. 20)
Figure 20: 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 21 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. 21) 22)
- 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.
Figure 21: 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. 23)
- 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.
Figure 22: 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. 24)
- 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.
Figure 23: 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. 25) 26)
- 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 24) 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.”
Figure 24: 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)
Figure 25: 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 25: 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. 27)
Figure 26: 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. 28) 29)
- 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.
Figure 27: 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.
Figure 28: 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. 30)
- 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.
Figure 29: 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. 31)
- “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.”
Figure 30: 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.”
Figure 31: 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. 32)
- 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 32: 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.
Figure 33: 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. 33)
- "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.
Table 3: Extended life for ESA's science missions 34)
• 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. 35) 36) 37)
- 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.
Figure 34: 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 35) emitted by the hot gas that pervades the galaxy cluster XLSSC006. 38)
- 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.
Figure 35: 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. 39)
- 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).
Figure 36: 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. 40) 41)
- 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.
Figure 37: 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
• 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. 42) 43)
Figure 38: 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)] 44)
- 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.
Figure 39: 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. 45) 46)
- 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 40: 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. 47)
• 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. 48)
Figure 41: 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?" 49)
- 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.
Figure 42: 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. 50) 51)
- 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.
Figure 43: 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.)
Figure 44: .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). 52)
- 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 45 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.
• 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. 53)
Figure 46: 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. 54)
- "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.
Figure 47: 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. 53)
• 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? 55)
- 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. 56) “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.”
Figure 48: 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)] 57)
- 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.
- 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. 58)
- 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 49 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.
Figure 49: 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)
• February 26, 2018: In 2014, ESA's XMM-Newton spotted X-rays emanating from the massive star Rho Ophiuchi A and, last year, found these to ebb and flow periodically in the form of intense flares – both unexpected results. The team has now used ESO's VLT (Very Large Telescope) to find that the star boasts a strong magnetic field, confirming its status as a cosmic lighthouse. 59)
Figure 50: This image from ESA's XMM-Newton space observatory shows a massive star named Rho Ophiuchi A. The star, visible at the center of the frame, sits at the heart of the Rho Ophiuchi Dark Cloud, a nearby region known to be actively forming new stars, located some 350 light years away (image credit: ESA/XMM-Newton; I. Pillitteri (INAF–Osservatorio Astronomico di Palermo))
- Stars like the Sun are known to produce strong X-ray flares, but massive stars appear to be very different. In stars upwards of eight solar masses X-ray emission is steady, and no such star had been confidently observed to repeatedly flare in this part of the spectrum – until recently.
In 2014, a team of scientists used ESA's XMM-Newton space observatory to observe a massive star named Rho Ophiuchi A. This star sits at the heart of the Rho Ophiuchi Dark Cloud, a nearby region known to be actively forming new stars. Surprisingly, the data showed an abundance of X-rays streaming out from the star, prompting the team to look closer.
"We observed the star with XMM-Newton for almost 40 hours and found something even more unexpected," says Ignazio Pillitteri of the INAF–Osservatorio Astronomico di Palermo, Italy, and leader of the research team.
"Rather than a smooth, steady emission, the X-rays pulsed periodically outwards from Rho Ophiuchi A, varying over a period of roughly 1.2 days as the star rotated – like an X-ray lighthouse! This is quite a new phenomenon in stars bigger than the Sun."
Figure 51: The flickering view of massive star Rho Ophiuchi A as observed by ESA's XMM-Newton space observatory in 2016 (image credit: ESA/XMM-Newton; I. Pillitteri (INAF–Osservatorio Astronomico di Palermo))
- Rho Ophiuchi A is far hotter and more massive than our parent star. It remains unknown how X-rays are generated in such stellar heavyweights; one possibility is a strong intrinsic magnetism, which would be observable via signs of surface magnetism. However, how such a magnetic field would come to be – and how it would be linked to any X-ray emission – remains unclear.
- "We guessed that there may be a giant active magnetic spot on the surface of Rho Ophiuchi A – a bit like a sunspot, only far bigger and more stable," adds Pillitteri. "As the star rotates, this spot would come in and out of view, causing the observed pulsing X-rays. However, this idea was somewhat unlikely; spots on stars form when an interior magnetic field pops out to the surface, and we know that only one in ten massive stars has a measurable magnetic field."
- Another way the pulsing 'lighthouse effect' could be created is via a lower-mass orbiting companion that added its own copious X-rays to the light attributed to Rho Ophiuchi A; this X-ray emission would vary in strength as the hypothetical smaller star crossed in front of and behind Rho Ophiuchi A during its 1.2-day orbit. The team also considered this possibility: that Rho Ophiuchi A could have a small, unseen, lower-mass companion in a very tight orbit.
- "To find out one way or another, we rushed to get magnetic measurements of Rho Ophiuchi A using one of the largest ground-based telescopes in existence: ESO's Very Large Telescope," says Lida Oskinova of the University of Potsdam, Germany, a member of the international team that conducted the study. "Excitingly, these measurements confirmed one of our predictions and showed that the X-rays are most likely linked to magnetic structures on the surface of the star."
- These measurements were made in visible light using a technique known as spectropolarimetry, which involves studying various wavelengths of polarized light emanating from a star. The data showed Rho Ophiuchi A to have an intense magnetic field some 500 times stronger than that of the Sun. 60) 61) 62)
- "Such a strong field is easily capable of producing the kind of flares we spotted," says Pillitteri. "This confirms that what we discovered using XMM-Newton were indeed X-ray flares on Rho Ophiuchi A, that massive stars can be magnetically active – as shown by the optical observations – and that this activity can be seen in X-rays."
- The combined data indicate that Rho Ophiuchi A is the only star of its type to have a confirmed active magnetic region on its surface that emits X-rays. Hunting for similar behavior in stars like Rho Ophiuchi A will help scientists to understand how prevalent this phenomenon is, and unravel more about the magnetic properties of such stars.
- "This study is an important one in our exploration of massive stars – there's much we still don't understand about these objects," says Norbert Schartel, ESA XMM-Newton Project Scientist. "Together, the extraordinary capabilities of XMM-Newton and the Very Large Telescope have now uncovered another piece of the puzzle."
- "As a bonus, it illustrates the process of science very well – of finding something interesting or unusual, investigating and coming up with a few possible hypotheses, and following up with more observation to figure out which is correct. It's a wonderful example of an international collaboration between telescopes, both in orbit and on the ground, working together to explore and explain the phenomena we see throughout the cosmos."
• February 2, 2018: ESA’s XMM-Newton has spotted surprising changes in the powerful streams of gas from two massive stars, suggesting that colliding stellar winds don’t behave as expected. 63)
- Massive stars – several times larger than our Sun – lead turbulent lives, burning their nuclear fuel rapidly and pouring large amounts of material into their surroundings throughout their short but sparkling lives.
- These fierce stellar winds can carry the equivalent of Earth’s mass in a month and travel at millions of km/hour, so when two such winds collide they unleash enormous amounts of energy. — The cosmic clash heats the gas to millions of degrees, making it shine brightly in X-rays.
Legend to Figure 52: Consisting of two huge stars each 60 times the mass of our Sun, HD 5980 was reported as the first stellar system with colliding winds to be discovered beyond our Milky Way galaxy in 2007. It is located in the Small Magellanic Cloud. The XMM-Newton data collected between 2000 and 2005 show a bright and energetic X-ray source, with variations characteristic of an ongoing interaction between the winds blown by the two stars. The system was expected to gently fade out over the years, but further observations performed with XMM-Newton in 2016 revealed the opposite: the system was two and a half times brighter than it had appeared a decade earlier, and its X-ray emission had shifted to higher energies.
- Normally, colliding winds change little because neither do the stars nor their orbits. However, some massive stars behave dramatically.
- This is the case with HD 5980, a pairing of two huge stars each 60 times the mass of our Sun and only about 100 million km apart – closer than we are to our star.
- One had a major outburst in 1994, reminiscent of the eruption that turned Eta Carinae into the second brightest star in the sky for about 18 years in the 19th century.
- While it is now too late to study Eta Carinae’s historic eruption, astronomers have been observing HD 5980 with X-ray telescopes to study the hot gas.
- In 2007, Yaël Nazé of the University of Liège, Belgium, and her colleagues discovered the collision of winds from these stars using observations made by ESA’s XMM-Newton and NASA’s Chandra X-ray telescopes between 2000 and 2005.
- Then they looked at it again with XMM-Newton in 2016. — “We expected HD 5980 to fade gently over the years as the erupting star settled back to normal – but to our surprise it did just the opposite,” says Yaël.
- They found the pair was two and a half times brighter than a decade earlier, and its X-ray emission was even more energetic.
- “We had never seen anything like that in a wind–wind collision.”
- With less material ejected but more light emitted, it was difficult to explain what was happening. — Finally, they found a theoretical study that offers a fitting scenario.
- “When stellar winds collide, the shocked material releases plenty of X-rays. However, if the hot matter radiates too much light, it rapidly cools, the shock becomes unstable and the X-ray emission dims. - This somewhat counterintuitive process is what we thought happened at the time of our first observations, more than 10 years ago. But by 2016, the shock had relaxed and the instabilities had diminished, allowing the X-ray emission to rise eventually.”
- These are the first observations that substantiate this previously hypothetical scenario. Yaël’s colleagues are now testing the new result in greater detail through computer simulations.
- “Unique discoveries like this demonstrate how XMM-Newton keeps providing astronomers with fresh material to improve our understanding of the most energetic processes in the Universe,” says Norbert Schartel, XMM-Newton project scientist at ESA.
Figure 53: A Hubble Space Telescope view of the cluster NGC 346 - the arrow indicates the position of HD 5980 [image credit: NASA, ESA, A. Nota (STScI/ESA)] 64)
Figure 54: ESA's XMM-Newton has spotted surprising changes in the powerful streams of gas from two massive stars in the binary star system HD 5980. One of the two stars had a major outburst reminiscent of the 19th-century eruption of Eta Carinae, and astronomers expected that its X-ray emission would fade gently over the years. Instead, they found the pair was two and a half times brighter than a decade earlier, and its X-ray emission was even more energetic, suggesting that colliding stellar winds don’t behave as expected (video credit: ESA) 65)
• December 11, 2017: A young massive star that began life around 25 times more massive than our own Sun is shedding shells of material and fast winds to create this dynamic scene captured by ESA’s XMM-Newton (Figure 55). 66)
- The star will likely end its life in a violent supernova explosion.
- The Crescent Nebula sits in the constellation of Cygnus about 5000 light-years away, exactly at a location in the sky that has not been accessible to XMM-Newton until recently. Although it has been well studied by other X-ray telescopes, astronomers working on XMM-Newton, which was launched on 10 December 1999, had to wait patiently until the orbit of the satellite was such that this patch of sky moved into its field of view in April 2014.
- More information about XMM-Newton’s observation is available in “X-ray emission from the Wolf-Rayet bubble NGC 688. II. XMM-Newton EPIC observations,” by J. Toalá et al. (2015).
Figure 55: The image shows the detailed structure of the Crescent Nebula that shed a shell of material as it expanded into a red giant some 200 000 years ago. Fast winds emitted more recently have now collided with that material, causing the gasses in the bubble to heat up and emit X-rays, seen as blue in the image. Other features can also be seen, such as the green hue, generated by oxygen atoms, where the star’s wind is interacting with the surrounding interstellar medium. Density differences in the surrounding material may give rise to the different structures, such as the extended bubble segment to the top right (image credit: ESA/XMM-Newton, J. Toalá & D. Goldman)
• December 7, 2017: ESA's SPC (Science Program Committee) has approved indicative extensions, up to 2019-2020, for the operation of eight scientific missions. 67)
- During its meeting at ESA Headquarters in Paris, on 21-22 November, the SPC approved indicative extensions for the continued operation of five ESA-led missions: Gaia, INTEGRAL, Mars Express, SOHO, and XMM-Newton. This followed a comprehensive review of the current operational status and outlook of the missions and their expected scientific returns during the extension period. The decision will be subject to confirmation towards the end of 2018.
- The lifetime of Gaia, ESA's billion star surveyor, was extended by eighteen months, from 25 July 2019 to 31 December 2020. This is the first time that Gaia, which was launched in 2013 and originally funded for a five-year mission, has been subject to the extension process. — Mars Express, SOHO, and XMM-Newton each received extensions of two years, so their operations will continue at least until the end of 2020.
- The SPC extended the operations of the high-energy observatory INTEGRAL by one year, until 31 December 2019. A proposal to extend INTEGRAL until the end of 2020, as well as a proposal concerning a two-year extension of the magnetospheric plasma mission, Cluster, will be presented to the next meeting of the SPC in February 2018.
- The go-ahead was also given to continue ESA's contributions to the operations of three international collaborative missions: the HST (Hubble Space Telescope), and IRIS (Interface Region Imaging Spectrograph), which are both led by NASA, as well as the Japanese-led mission Hinode.
- 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. Extensions for operations in the period 2017-2018 were approved by the SPC in November 2016, but the indicative extension, for 2019-2020, had been deferred until the November 2017 meeting to allow the SPC to evaluate the outcome of the ESA Ministerial Council meeting in December 2016.
• October 30, 2017: ESA and NASA space telescopes have revealed that, unlike Earth’s polar lights, the intense auroras seen at Jupiter’s poles unexpectedly behave independently of one another. 68)
- Auroras have been seen in many places, from planets and moons to stars, brown dwarfs and a variety of other cosmic bodies. These beautiful displays are caused by streams of electrically charged atomic particles – electrons and ions – colliding with the atmospheric layers surrounding a planet, moon or star. Earth’s polar lights tend to mirror one another: when they brighten at the North pole, they generally brighten at the South pole, too.
- The same was expected of auroras elsewhere, but a new study, published today in Nature Astronomy, reveals that those at the gas giant Jupiter are much less coordinated.
- The study used ESA’s XMM-Newton and NASA’s Chandra X-ray space observatories to observe the high-energy X-rays produced by the auroras at Jupiter’s poles. While the southern auroras were found to pulse consistently every 11 minutes, those at the planet’s north pole flared chaotically. 69)
- “These auroras don’t seem to act in unison like those that we’re often familiar with here on Earth,” says lead author William Dunn of University College London’s MSSL (Mullard Space Science Laboratory), UK, and Harvard-Smithsonian Center for Astrophysics, USA.
- "We thought the activity would be coordinated through Jupiter's magnetic field, but the behavior we found is really puzzling. It's stranger still considering that Saturn – another gas giant planet – doesn't produce any X-ray auroras that we can detect, so this throws up a couple of questions that we're currently unsure how to answer. Firstly, how does Jupiter produce bright and energetic X-ray auroras at all when its neighbor doesn't, and secondly, how does it do so independently at each pole?"
- With the data at hand, William Dunn and colleagues identified and mapped X-ray hot spots at Jupiter's poles. Each hot spot covers an area half the size of Earth's surface.
- As well as raising questions about how auroras are produced throughout the cosmos, Jupiter's independently pulsing auroras suggest that there is far more to understand about how the planet itself produces some of its most energetic emissions.
- Jupiter's magnetic influence is colossal; the region of space over which the Jovian magnetic field dominates – the magnetosphere – is some 40 times larger than Earth's, and filled with high-energy plasma. In the outer edges of this region, charged particles ultimately from volcanic eruptions on Jupiter's moon, Io, interact with the magnetic boundary between the magnetosphere and interplanetary space. These interactions create intense phenomena, including auroras.
- "Charged particles have to hit Jupiter's atmosphere at exceptionally fast speeds in order to generate the X-ray pulses that we've seen. We don't yet understand what processes cause this, but these observations tell us that they act independently in the northern and southern hemispheres," adds Licia Ray, from Lancaster University, UK, and a co-author.
- The asymmetry in Jupiter's northern and southern lights also suggests that many cosmic bodies that are known to experience auroras – exoplanets, neutron stars, brown dwarfs and other rapidly-rotating bodies – might produce a very different aurora at each pole.
- Further studies of Jupiter's auroras will help to form a clearer picture of the phenomena produced at Jupiter; auroral observing campaigns are planned for the next two years, with X-ray monitoring by XMM-Newton and Chandra and simultaneous observations from NASA's Juno, a spacecraft that started orbiting Jupiter in mid-2016.
- ESA's JUICE (JUpiter ICy moons Explorer) will arrive at the planet by 2029, to investigate Jupiter's atmosphere and magnetosphere. It, too, will observe the auroras and in particular the effect on them of the Galilean moons.
- "This is a breakthrough finding, and it couldn't have been done without ESA's XMM-Newton," adds Norbert Schartel, ESA project scientist for XMM-Newton. "The space observatory was critical to this study, providing detailed data at a high spectral resolution such that the team could explore the vibrant colors of the auroras and figure out details about the particles involved: if they're moving fast, whether they're an oxygen or sulphur ion, and so on. Coordinated observations like these, with telescopes such as XMM-Newton, Chandra and Juno working together, are key in exploring and further understanding environments and phenomena across the Universe, and the processes that produce them."
Legend to Figure 56: Jupiter experiences intense auroras that rage constantly, created as charged particles from the Sun and the Jovian moon Io stream towards the gas giant and interact with its atmosphere. - A new study has revealed that the planet's northern and southern lights behave and pulse independently of one another–an unexpected finding given the behavior of Earth's auroras, which tend to mirror one another (when they brighten at the North pole, they generally brighten at the South pole too). - This image, from the NASA/ESA Hubble Space Telescope, shows such an aurora on Jupiter. It is a composite of two separate sets of Hubble observations–one of the polar aurora itself, photographed in the ultraviolet in June 2016, and an optical view of the full disc of Jupiter, taken in April 2014.
• September 6, 2017: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to just how hospitable a star system could be. A team of researchers looked at 24 stars similar to the Sun, each at least one billion years old, and how their X-ray brightness changed over time. 70) 71) 72)
- High levels of magnetic activity can produce bright X-rays and ultraviolet light from stellar flares. Strong magnetic activity can also generate powerful eruptions of material from the star's surface. Such energetic radiation and eruptions can impact planets and could damage or destroy their atmospheres, as pointed out in previous studies, including Chandra work reported in 2011 and 2013.
- Since stellar X-rays mirror magnetic activity, X-ray observations can tell astronomers about the high-energy environment around the star. In the new study the X-ray data from Chandra and XMM-Newton revealed that stars like the Sun and their less massive cousins calm down surprisingly quickly after a turbulent youth.
- “This is good news for the future habitability of planets orbiting Sun-like stars, because the amount of harmful X-rays and ultraviolet radiation striking these worlds from stellar flares would be less than we used to think,” said Rachel Booth, a graduate student at Queen’s University in Belfast, UK, who led the study.
- To understand how quickly stellar magnetic activity level changes over time, astronomers need accurate ages for many different stars. This is a difficult task, but new precise age estimates have recently become available from studies of the way that a star pulsates using NASA’s Kepler and ESA’s CoRoT missions. These new age estimates were used for most of the 24 stars studied here.
Legend to Figure 57: This artist's illustration depicts one of these comparatively calm, older Sun-like stars with a planet in orbit around it. The large dark area is a "coronal hole", a phenomenon associated with low levels of magnetic activity. The inset box shows the Chandra data of one of the observed objects, a two billion year old star called GJ 176, located 30 light years from Earth.
• July 10, 2017: Highly obscured and rapidly growing SMBH (Super-Massive Black Holes), known as AGN (Active Galactic Nuclei), might represent the key phase when SMBH accreted most of their mass and when the relationship between galaxies and their central SMBHs was established. A new study by an international team of astronomers led by Silvia Mateos from the Instituto de Física de Cantabria (CSIC-UC) in Spain, now suggests that many of the brightest SMBH may be escaping our detection as they hide in heavily obscured environments. 73)
- A large observational effort has been devoted to reveal the obscured AGN phase across the history of the Universe. Oddly enough, most AGN searches, mainly at X-ray wavelengths, report that obscured AGN (optically classified as type 2) become rarer with increasing accretion power. The receding torus model hypothesis is often invoked to explain this lack of luminous highly obscured AGN. It postulates that the most luminous AGN erode the obscuring material located in the vicinity of the SMBH, the so-called "dusty torus", reducing its covering factor. As a result, higher luminosity AGN should be rarely hidden from our view by the dusty torus.
- A completely different picture emerges now. Based on observations from both ground and space instruments, including spectroscopic observations with ISIS and ACAM on the William Herschel Telescope (WHT) and the European Photon Imaging Camera onboard the XMM-Newton observatory, the team led by Mateos built a sample of approximately 200 X-ray selected AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). Using models that self-consistently reproduce the emission from dust in the torus heated by the AGN, they determined for each AGN in their sample the fraction of the sky around the AGN central engine that is obscured, i.e., the geometrical covering factor of the dusty torus. The combined torus covering factor over the entire AGN population is precisely the intrinsic fraction of type 2 AGN.
- By equating the distribution of torus covering factors to the observed fraction of type 2 AGN in BUXS the team finds that many type 2 AGN with high covering factor tori have escaped detection in X-rays. Most importantly, these elusive AGN are increasingly numerous at higher AGN luminosities. When they account for the "missing" AGN, the team finds an intrinsic fraction of type 2 AGN of 58% with no (or at most a weak) dependence with AGN luminosity over three orders of magnitude in luminosity.
- According to Silvia Mateos, who led the research: "Clearly, the impact of the enormous AGN accretion power on the geometry of the nuclear material surrounding the SMBH is not as strong as we thought. Instead, a large fraction of luminous rapidly growing SMBH are so deeply embedded that they are escaping X-ray detection". 74)
Figure 58: Type-2 AGN fraction vs. torus covering factor f2 for objects with 1042 <LX <1043 (top), 1043 <LX < 1044 (middle), and 1044 LX < 1045 erg s-1 (bottom). Filled circles are the observed type-2 AGN fractions in BUXS. Open squares are the best-fit models to the 1:1 relations (black solid lines) obtained by allowing a population of non-detected type-2 sources. The insets show the assumed f2 distribution of these missed sources (image credit: International Study Team)
• June 6, 2017: This blue ‘ball of strings’ actually records 2114 movements made by ESA’s XMM-Newton space telescope as it shifted its gaze from one X-ray object to another between August 2001 and December 2014 (Figure 59). — Orbiting in space since 1999, XMM-Newton is studying high-energy phenomena in the Universe, such as black holes, neutron stars, pulsars and stellar winds. 75)
- Even when moving its focus between objects, the space telescope collects scientific data, revealing X-ray sources across the entire sky. After correcting for overlaps between slews, 84% of the sky has now been covered.
- The plot is in galactic coordinates such that the center of the plot corresponds to the center of the Milky Way. The slew paths pass predominantly through the ecliptic poles, indicated by the density of overlapping slew paths to the top left and bottom right.
- Over 5000 papers have been published on XMM-Newton results to date. Scientists are also looking forward to the next generation of X-ray satellite, such as ESA’s ATHENA (Advanced Telescope for High-ENergy Astrophysics), which is expected to be launched towards the end of the next decade.
Figure 59: The image was created as part of the XMM-Newton Slew Survey Catalog release in March 2017, and which was featured as our Space Science Image of the Week last month (image credit: ESA/XMM-Newton/A. Read/R. Saxton , CC BY-SA 3.0 IGO)
• May 15, 2017: This colorful, seemingly abstract artwork is actually a map (Figure 60) depicting all the celestial objects that were detected in the XMM-Newton slew survey between August 2001 and December 2014. 76)
- Orbiting Earth since 1999, XMM-Newton is studying high-energy phenomena in the Universe, such as black holes, neutron stars, pulsars and stellar winds. But even when moving between specific targets, the space telescope collects scientific data.
- The plot is in galactic coordinates such that the center of the plot corresponds to the center of the Milky Way. High-energy sources along the center of the Milky Way include the famous black hole Cygnus X-1, and Vela X-1, a binary system comprising a neutron star consuming matter from a supergiant companion.
- Several star-and-black hole binary systems are also captured, including objects identified as GRS 1915+105, 4U 1630-47 and V 4641 Sgr.
- Two clusters of sources, one to the top left and one to the bottom right, correspond to the ecliptic poles. Objects above and below the plane of our Galaxy are predominantly external galaxies that are emitting X-rays from their massive black holes.
Figure 60: The map shows the 30,000 sources captured during 2114 of these slews. Because of overlapping slew paths, some sources have been observed up to 15 times, and 4924 sources have been observed twice or more. After correcting for overlaps between slews, 84% of the sky has been covered. The plot is color-coded such that sources of a lower energy are red and those with a higher energy are blue. In addition, the brighter the source, the larger it appears on the map (image credit: ESA/XMM-Newton / R. Saxton / A.M. Read)
• May 10, 2017, Astronomers produced this dramatic new, highly-detailed image of the Crab Nebula (Figure 61) by combining data from telescopes spanning nearly the entire breadth of the electromagnetic spectrum, from the long waves seen by the Karl G. Jansky VLA (Very Large Array) to the extremely short waves seen by the orbiting Chandra X-Ray Observatory. 77) 78)
- The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, is some 6,500 light-years from Earth. At its center is a superdense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams of radio waves and light — a pulsar. The nebula’s intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.
- The new VLA, Hubble, and Chandra observations all were made at nearly the same time in November of 2012. A team of scientists led by Gloria Dubner of IAFE, the National Council of Scientific Research (CONICET), and the University of Buenos Aires in Argentina then made a detailed analysis of the newly-revealed details in a quest to gain new insights into the complex physics of the object. They are reporting their findings in the Astrophysical Journal. 79)
- New details from the study show interactions between fast-moving particles and magnetic fields similar to structures seen on the Sun, other features seen to appear at multiple wavelengths, and structures that may indicate features near the star before it exploded. Two separate jets of material from near the pulsar appear in the X-ray and the radio images.
- “Comparing these new images, made at different wavelengths, is providing us with a wealth of new detail about the Crab Nebula. Though the Crab has been studied extensively for years, we still have much to learn about it,” Dubner said.
- The NRAO (National Radio Astronomy Observatory) is a facility of the NSF (National Science Foundation), operated under cooperative agreement by Associated Universities, Inc.
Figure 61: This image combines data from five different telescopes: The VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-Ray Observatory (X-ray) in purple (image credit: G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; NRAO/AUI/NSF; A. Loll et al.; T. Temim et al.; F. Seward et al.; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI)
• March 1, 2017: ESA and NASA space telescopes have made the most detailed observation of an ultra-fast wind flowing from the vicinity of a black hole at nearly a quarter of the speed of light. Outflowing gas is a common feature of the supermassive black holes that reside in the center of large galaxies. Millions to billions of times more massive than the Sun, these black holes feed off the surrounding gas that swirls around them. Space telescopes see this as bright emissions, including X-rays, from the innermost part of the disc around the black hole. 80)
- Occasionally, the black holes eat too much and burp out an ultra-fast wind. These winds are an important characteristic to study because they could have a strong influence on regulating the growth of the host galaxy by clearing the surrounding gas away and therefore suppressing the birth of stars.
- Using ESA’s XMM-Newton and NASA’s NuSTAR telescopes, scientists have now made the most detailed observation yet of such an outflow, coming from an active galaxy identified as IRAS 13224–3809. The winds recorded from the black hole reach 71 000 km/s – 0.24 times the speed of light – putting it in the top 5% of fastest known black hole winds.
- XMM-Newton focused on the black hole for 17 days straight, revealing the extremely variable nature of the winds. “We often only have one observation of a particular object, then several months or even years later we observe it again and see if there’s been a change,” says Michael Parker of the Institute of Astronomy at Cambridge, UK, lead author of the paper published in Nature this week that describes the new result. ”Thanks to this long observation campaign, we observed changes in the winds on a timescale of less than an hour for the first time.” 81)
- The changes were seen in the increasing temperature of the winds, a signature of their response to greater X-ray emission from the disc right next to the black hole.
- Furthermore, the observations also revealed changes to the chemical fingerprints of the outflowing gas: as the X-ray emission increased, it stripped electrons in the wind from their atoms, erasing the wind signatures seen in the data. “The chemical fingerprints of the wind changed with the strength of the X-rays in less than an hour, hundreds of times faster than ever seen before,” says co-author Andrew Fabian, also from the Institute of Astronomy and principal investigator of the project. “It allows us to link the X-ray emission arising from the infalling material into the black hole, to the variability of the outflowing wind farther away.”
- “Finding such variability, and finding evidence for this link, is a key step in understanding how black hole winds are launched and accelerated, which in turn is an essential part of understanding their ability to moderate star formation in the host galaxy,” adds Norbert Schartel, ESA’s XMM-Newton project scientist.
- The brightness of an active galactic nucleus is set by the gas falling onto it from the galaxy, and the gas infall rate is regulated by the brightness of the active galactic nucleus; this feedback loop is the process by which supermassive black holes in the centers of galaxies may moderate the growth of their hosts.
Figure 62: Artist impression illustrating a supermassive black hole with X-ray emission emanating from its inner region (pink) and ultrafast winds streaming from the surrounding disk (purple), image credit: ESA
• February 21, 2017: ESA’s XMM-Newton has found a pulsar – the spinning remains of a once-massive star – that is a thousand times brighter than previously thought possible (Figure 63). The pulsar is also the most distant of its kind ever detected, with its light travelling 50 million light-years before being detected by XMM-Newton. 82) 83)
- Pulsars are spinning, magnetized neutron stars that sweep regular pulses of radiation in two symmetrical beams across the cosmos. If suitably aligned with Earth these beams are like a lighthouse beacon appearing to flash on and off as it rotates. They were once massive stars that exploded as a powerful supernova at the end of their natural life, before becoming small and extraordinarily dense stellar corpses.
- This X-ray source is the most luminous of its type detected to date: it is 10 times brighter than the previous record holder. In one second it emits the same amount of energy released by our Sun in 3.5 years.
- XMM-Newton observed the object several times in the last 13 years, with the discovery a result of a systematic search for pulsars in the data archive – its 1.13 s periodic pulses giving it away. The signal was also identified in NASA’s NuSTAR archive data, providing additional information.
- “Before, it was believed that only black holes at least 10 times more massive than our Sun feeding off their stellar companions, could achieve such extraordinary luminosities, but the rapid and regular pulsations of this source are the fingerprints of neutron stars and clearly distinguish them from black holes,” says Gian Luca Israel, from the INAF-Osservatorio Astronomica di Roma, Italy, lead author of the paper describing the result published in Science this week.
- The archival data also revealed that the pulsar’s spin rate has changed over time, from 1.43 s per rotation in 2003 to 1.13 s in 2014. The same relative acceleration in Earth’s rotation would shorten a day by five hours in the same time span. “Only a neutron star is compact enough to keep itself together while rotating so fast,” adds Gian Luca.
- Although it is not unusual for the rotation rate of a neutron star to change, the high rate of change in this case is likely linked to the object rapidly consuming mass from a companion. “This object is really challenging our current understanding of the ‘accretion’ process for high-luminosity stars,” says Gian Luca. “It is 1000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object.”
- The scientists think there must be a strong, complex magnetic field close to its surface, such that accretion onto the neutron star surface is still possible while still generating the high luminosity.
- “The discovery of this very unusual object, by far the most extreme ever discovered in terms of distance, luminosity and rate of increase of its rotation frequency, sets a new record for XMM-Newton, and is changing our ideas of how such objects really ‘work’,” says Norbert Schartel, ESA’s XMM-Newton project scientist.
Figure 63: The record-breaking pulsar, identified as NGC 5907 X-1, is in the spiral galaxy NGC 5907, which is also known as the Knife Edge Galaxy or Splinter Galaxy. The image comprises X-ray emission data (blue/white) from ESA’s XMM-Newton space telescope and NASA’s Chandra X-ray observatory, and optical data from the Sloan Digital Sky Survey (galaxy and foreground stars), image credit: ESA/XMM-Newton; NASA/Chandra and SDSS
Legend to Figure 63: The inset shows the X-ray pulsation of the spinning neutron star, which has a period of 1.13 s, as determined by XMM-Newton’s European Photon Imaging Camera.
• January 31, 2017: Scientists observing a curious neutron star in a binary system known as the 'Rapid Burster' may have solved a forty-year-old mystery surrounding its puzzling X-ray bursts. They discovered that its magnetic field creates a gap around the star, largely preventing it from feeding on matter from its stellar companion. Gas builds up until, under certain conditions, it hits the neutron star all at once, producing intense flashes of X-rays. The discovery was made with space telescopes including ESA's XMM-Newton. 84)
- Discovered in the 1970s, the Rapid Burster is a binary system comprising a low-mass star in its prime and a neutron star – the compact remnant of a massive star's demise. In such a stellar pair, the gravitational pull of the dense remnant strips the other star of some of its gas; the gas forms an accretion disc and spirals towards the neutron star.
- As a result of this accretion process, most neutron star binaries continuously release large amounts of X-rays, which are punctuated by additional X-ray flashes every few hours or days. Scientists can account for these 'type-I' bursts, in terms of nuclear reactions that are ignited in the inflowing gas – mainly hydrogen – when it accumulates on the neutron star's surface.
- But the Rapid Burster is a peculiar source: at its brightest, it does emit these type-I flashes, while during periods of lower X-ray emission, it exhibits the much more elusive 'type-II' bursts – these are sudden, erratic and extremely intense releases of X-rays.
- In contrast to type-I bursts, which do not represent a significant release of energy with respect to what is normally emitted by the accreting neutron star, bursts of type-II liberate enormous amounts of energy during periods otherwise characterized by very little emission occurring (the relative energy output of a burst with respect to the normal accretion process is tens to hundreds of times higher in type-II bursts than in type-I bursts).
- Despite forty years of searches, type-II bursts have been detected only in one other source besides the Rapid Burster. Known as the Bursting Pulsar and discovered in the 1990s, this binary system comprises a low-mass star and a highly magnetized, spinning neutron star – a pulsar – that exhibits only type-II bursts.
- Because of the scarcity of sources that display this phenomenon, the underlying physical mechanisms have long been debated, but a new study of the Rapid Burster provides first evidence for what is occurring.
- "The Rapid Burster is the archetypal system to investigate type-II bursts – it's where they were first observed and the only source that shows both type-I and type-II bursts," says Jakob van den Eijnden, a PhD student at the Anton Pannekoek Institute for Astronomy in Amsterdam, The Netherlands, and lead author of a Letter published in Monthly Notices of the Royal Astronomical Society. 85) — In this study, Jakob and his colleagues organized an observing campaign using three X-ray space telescopes to find out more about this system.
- Under the coordination of co-author Tullio Bagnoli, who was also based at the Anton Pannekoek Institute for Astronomy, the team managed to observe the source bursting over a few days in October 2015 with a combination of NASA's NuSTAR and Swift, and ESA's XMM-Newton.
Figure 64: Variations of brightness observed in the binary system MXB 1730-335, also known as the 'Rapid Burster' as observed by NASA's NuSTAR X-ray telescope (image credit: NASA, adapted from Ref. 85) 86)
- They first monitored the source with Swift, timing the observations for a period when they expected a series of type-II bursts to take place. Then, soon after the first burst was detected, the scientists set the other observatories into motion, using XMM-Newton to measure X-rays emitted directly by the neutron star's surface or by gas in the accretion disc, and NuSTAR to detect higher-energy X-rays, which are emitted by the neutron star and reflected off the disc. - With these data, the scientists scrutinized the structure of the accretion disc to understand what happens to it before, during, and after these copious releases of X-rays.
- According to one model, type-II bursts occur because the fast spinning magnetic field of the neutron star keeps the gas flowing from the companion star at bay, preventing it from reaching closer to the neutron star and effectively creating an inner edge at the center of the disc. However, as the gas continues to flow and accumulate near this edge, it spins faster and faster, and eventually catches up with the spinning velocity of the magnetic field.
- "It's as if we threw something towards a merry-go-round that is spinning very fast: it would bounce off, unless it's thrown at the same velocity as the machine," explains Jakob. "A similar balancing act happens between the inflowing gas and the spinning magnetic field: as long as the gas hasn't the right speed, it cannot get to the neutron star and it can only pile up at the edge. By the time it reaches the right velocity, a lot of gas has accumulated and it hits the neutron star all at once, giving rise to the dramatic emission of type-II bursts."
- This model predicts that, while the material is piling up, a gap should form between the neutron star and the edge of the accretion disc.
Figure 65: Artist's impression of the neutron star in the Rapid Burster (image credit: ESA/ATG medialab)
- In other models, the intense flashes are explained as arising from instabilities in the flow of the accreting gas or from general-relativistic effects. In either case, these would take place much closer to the neutron star and not give rise to such a gap. "A gap is exactly what we found at the Rapid Burster," says Nathalie Degenaar, a researcher at Anton Pannekoek Institute for Astronomy and Jakob's PhD advisor. "This strongly suggests that the type-II bursts are caused by the magnetic field."
- The observations indicate that there is a gap of roughly 90 km between the neutron star and the inner edge of the accretion disc. While not impressive on cosmic scales, the size of the gap is much larger than the neutron star itself, which has a radius of about 10 km.
- This finding is in line with results from a previous study by Nathalie and collaborators, who had observed a similar gap around the Bursting Pulsar – the other source known to produce type-II bursts.
- In the new study of the Rapid Burster, the scientists also measured the strength of the neutron star's magnetic field: at 6 x 108 G (gauss), it is around a billion times stronger than Earth's and, most important, over five times stronger than observed in other neutron stars with a low-mass stellar companion. This could hint at a young age of this binary system, suggesting that the accretion process has not been going on for long enough to damp the magnetic field down, as is thought to have happened in similar systems.
- If this neutron star binary really is as young as its strong magnetic field indicates, then it is expected to spin much slower than its older counterparts: future measurements of the star's spinning rate might help confirm this unusual scenario.
- "This result is a big step towards solving a forty-year-old puzzle in neutron star astronomy, while also revealing new details about the interaction between magnetic fields and accretion discs in these exotic objects," concludes Norbert Schartel, XMM-Newton Project Scientist at ESA.
• December 19, 2016: An important part of studying celestial objects is understanding and removing the background noise. The image presented in Figure 66 was created to demonstrate the power of software tools used to analyze observations by ESA’s XMM-Newton of large objects like galaxies, clusters of galaxies, and supernova remnants. The tool models and subtracts the background noise, which is very difficult for large, fuzzy objects like these, and to create exposure-corrected images. It also merges and smooths the observations taken of an individual object by XMM-Newton’s three X-ray cameras of the EPIC (European Photon Imaging Camera) instrument, and to allow the mosaicking of multiple observations. 87)
- It certainly does not look like it, but the star-like feature on the right corresponds to spiral galaxy M81. Similarly, the feature on the left arises from the Holmberg IX dwarf galaxy.
- By examining images like these, along with complementary images taken at other wavelengths, scientists can get a quick look at the structure of the object and the spectral variations across the field. The structure of the bright patterns contains information about the origin of the emission, such as whether it comes from a ‘halo’ around the galaxy, or is confined to the disc and arms.
- For example, this particular image shows that there is a bright point source at the center of M81, resulting from the galaxy’s active core. There is also a decrease of brightness away from the central source, with fainter extended emission around it. Another galaxy might display brighter emission along its spiral arms.
- Bright emission is also apparent from the X-ray source in the dwarf galaxy. The ‘rays’ extending from the point sources are artefacts, seen whenever there is a very bright point source in the field of view. But even artefacts can be beautiful.
- The XMM-Newton Extended Source Analysis Software package (XMM-ESAS) was developed at the NASA/GSFC ( Goddard Space Flight Center) XMM-Newton GOF (Guest Observer Facility) in cooperation with the XMM-Newton Science Operations Center and the Background Working Group.
• December 8, 2016: XMM-Newton is one of Europe’s longest-flying and most productive orbiting observatories, investigating the hot X-ray Universe. Thanks to teamwork and technical innovation, it’s on track to keep flying for a long time yet. Launched 17 years ago, ESA’s orbiting X-ray telescope has helped scientists around the world to understand some of our Universe’s most mysterious events, from what happens in and around black holes to how galaxies formed. At 3800 kg, the 10 m-long XMM-Newton is the biggest science satellite ever built in Europe and its telescope mirrors are the most sensitive ever developed.-Expected to operate for as long as a decade, the hardy spacecraft has happily surprised everyone by lasting almost two decades – and it shows no signs of giving up. 88)
- The success of XMM-Newton has been possible not only because of the robust spacecraft, but also the close cooperation between ESA’s astronomy center near Madrid, Spain, and the mission controllers at ESA’s operations center in Darmstadt, Germany.
- “The total number of 4775 scientific publications to date, with 358 from this year alone, is an impressive record of the mission’s scientific success, covering many, many areas of astrophysics,” notes project scientist Norbert Schartel.
- But keeping it fit and healthy into its third decade means the team must continue to develop and test new control techniques. A complex change to the orbit control system has almost halved fuel consumption, for example.
- (Not) running on empty: For starters, keeping XMM in orbit will require occasional thruster firings, about once per day, and that means burning fuel. “We’ve got plenty of fuel and over the years we’ve figured out how to use less and less to maintain our science orbit,” says Marcus Kirsch, spacecraft operations manager. “The fuel is distributed between four separate tanks, but the main tank will run dry first. The design means we could not use the remaining fuel in the other tanks, so we're moving it all into tank 1. This will enable us to continue operations into the coming decade.”
- Back in the control room: As part of this process, the flight control team returned to ESA’s large, general-purpose Main Control Room at mission control in November – the first time since launch in 1999 – for five days of intensive simulations. The team usually works from a smaller, dedicated room shared with the Integral and Gaia mission teams. — The simulations checked the procedures that will be used for moving the fuel and for reconfiguring XMM for working beyond 2017.
- No one's ever done this before: “Not many spacecraft use the specially designed tank fuel system like on XMM,” says Nikolai von Krusenstiern, spacecraft operations engineer. - “As far as we know, no one’s ever shifted fuel from one tank to another with a tank design like ours in a satellite in orbit, and we want to take as much time as necessary to minimize any risk to the mission.” Tank-to-tank replenishment was never foreseen in the original design specifications – as XMM wasn’t meant to last so long – so no process was devised by builder Astrium (now Airbus Defence & Space). “Airbus have been very helpful – they even helped us get in touch with the now-retired designer of the fuel system to help us to safely design the procedures,” says Nikolai.
- XMM's third decade: The team will now analyze results of last month’s simulations with the aim of reconfiguring the spacecraft in 2017. This will complement the careful optimization of the flight control procedures already in place, and keep XMM thrusters firing – and the spacecraft flying reliably – into 2023. After that, the team will have a low-risk and confirmed plan on hand to conduct the fuel replenishment, which would thereafter keep the craft in its science mission well into its third decade. “The time spent in training and simulations last month was hugely valuable for the entire team,” says Marcus. “We worked together to devise a solid solution for XMM’s coming decades, and the individual engineers gained excellent training experience that they can use for XMM or even take with them if assigned to other missions.”