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. 147).
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)
• July 28, 2021: For the first time, astronomers have seen light coming from behind a black hole. 16)
- Using ESA’s XMM-Newton and NASA’s NuSTAR space telescopes, an international team of scientists led by Dan Wilkins of Stanford University in the USA observed extremely bright flares of X-ray light coming from around a black hole.
Figure 16: The X-ray flares echoed off of the gas falling into the black hole, and as the flares were subsiding, the telescopes picked up fainter flashes, which were the echoes of the flares bouncing off the gas behind the black hole. This supermassive black hole is 10 million times as massive as our Sun and located in the centre of a nearby spiral galaxy called I Zwicky 1, 800 million light-years away from Earth (image credit: ESA)
- The astronomers did not expect to see anything from behind the black hole, since no light can escape from it. But because of the black hole’s extreme gravity warping the space around it, light echoes from behind the black hole were bent around the black hole, making them visible from XMM and NuSTAR’s point of view.
- The discovery began with the search to find out more about the mysterious ‘corona’ of the black hole, which is the source of the bright X-ray light. Astronomers think that the corona is a result of gas that falls continuously into the black hole, where it forms a spinning disk around it – like water flushing down a drain.
- This gas disk is heated up to millions of degrees and generates magnetic fields that get twisted into knots by the spinning black hole. When the magnetic field gets tied up, it eventually snaps, releasing the energy stored within it. This heats everything around it and produces the corona of high energy electrons that produce the X-ray light.
- The X-ray flare observed from I Zwicky 1 was so bright that some of the X-rays shone down onto the disk of gas falling into the black hole. The X-rays that reflected on the gas behind the black hole were bent around the black hole, and these smaller flashes arrived at the telescopes with a delay. These observations match Einstein’s predictions of how gravity bends light around black holes, as described in his theory of General Relativity.
- The echoes of X-rays from the disk have specific ‘colors’ of light and as the X-rays travel around the black hole, their colors change slightly. Because the X-ray echoes have different colors and are seen at different times, depending where on the disk they reflected from, they contain a lot of information about what is happening around a black hole. The astronomers want to use this technique to create a 3D map of the black hole surroundings.
- Another mystery to be solved in future studies is how the corona produces such bright X-ray flares. The mission to characterize and understand black hole coronas will continue with XMM-Newton and ESA’s future X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics).
- The team published their findings in Nature. 17)
• July 13, 2021: A puzzler about the gas giant’s intense northern and southern lights has been deciphered. 18)
Figure 17: The purple hues in this image show X-ray emissions from Jupiter’s auroras, detected by NASA’s Chandra Space Telescope in 2007. They are overlaid on an image of Jupiter taken by NASA’s Hubble Space Telescope. Jupiter is the only gas giant planet where scientists have detected X-ray auroras [image credits: (X-ray) NASA/CXC/SwRI/R. Gladstone et al.; (Optical) NASA/ESA/Hubble Heritage (AURA/STScI)]
- Planetary astronomers combined measurements taken by NASA’s Juno spacecraft orbiting Jupiter, with data from ESA’s (the European Space Agency’s) Earth-orbiting XMM-Newton mission, to solve a 40-year-old mystery about the origins of Jupiter’s unusual X-ray auroras. For the first time, they have seen the entire mechanism at work: The electrically charged atoms, or ions, responsible for the X-rays are “surfing” electromagnetic waves in Jupiter’s magnetic field down into the gas giant’s atmosphere.
- Auroras have been detected on seven planets in our solar system. Some of these light shows are visible to the human eye; others generate wavelengths of light we can only see with specialized telescopes. Shorter wavelengths require more energy to produce. Jupiter has the most powerful auroras in the solar system and is the only one of the four giant planets with an aurora that has been found to emit X-rays.
- Planetary astronomers have been fascinated with Jupiter’s X-ray auroral emission since its discovery four decades ago because it was not immediately clear how the energy required to produce it is generated. They knew these surprising Jovian northern and southern lights are triggered by ions crashing into Jupiter’s atmosphere. But until now scientists had no idea how the ions responsible for the X-ray light show are able to get to the atmosphere in the first place.
- At Earth, auroras are usually visible only in a belt surrounding the magnetic poles, between 65 and 80 degrees latitude. Beyond 80 degrees, auroral emission disappears because the magnetic field lines leave Earth and connect to the magnetic field in the solar wind, which is the constant flux of electrically charged particles ejected by the Sun. These are called open field lines, and in the traditional picture, Jupiter’s and Saturn’s high-latitude polar regions are not expected to emit substantial auroras, either.
- However, Jupiter’s X-ray auroras are different. They exist poleward of the main auroral belt and pulsate, and those at the north pole often differ from those at the south pole. These are typical features of a closed magnetic field, where the magnetic field line exits the planet at one pole and reconnects with the planet at the other. All planets with magnetic fields have both open and closed field components.
- However, Jupiter’s X-ray auroras are different. They exist poleward of the main auroral belt and pulsate, and those at the north pole often differ from those at the south pole. These are typical features of a closed magnetic field, where the magnetic field line exits the planet at one pole and reconnects with the planet at the other. All planets with magnetic fields have both open and closed field components.
- Scientists studying the phenomena turned to computer simulations and found that the pulsating X-ray auroras could be linked to closed magnetic fields that are generated inside Jupiter and then stretch out millions of miles into space before turning back. But how to prove the model was viable?
- The study authors turned to data acquired by both Juno and XMM-Newton from July 16 to 17, 2017. During the two-day span, XMM-Newton observed Jupiter continuously for 26 hours and saw X-ray aurora pulsating every 27 minutes.
- At the same time, Juno had been traveling between 62 and 68 Jupiter radii (about 2.8 to 3 million miles, or 4.4 to 4.8 million kilometers) above the planet’s pre-dawn area. This was exactly the region that the team’s simulations suggested was important for triggering the pulsations, so they searched the Juno data for any magnetic processes that were occurring at the same rate.
- They found that fluctuations of Jupiter’s magnetic field caused the pulsating X-ray auroras. The outer boundary of the magnetic field is struck directly by the particles of the solar wind and compressed. These compressions heat ions that are trapped in Jupiter’s extensive magnetic field, which are millions of miles away from the planet’s atmosphere.
- “What we see in the Juno data is this beautiful chain of events. We see the compression happen, we see the EMIC wave triggered, we see the ions, and then we see a pulse of ions traveling along the field line,” said William Dunn of the Mullard Space Science Laboratory, University College London, and a co-author of the paper. “Then, a few minutes later, XMM sees a burst of X-rays.”
- Now that the missing piece of the process has been identified for the first time, it opens up a wealth of possibilities for where it could be studied next. For example, at Jupiter, the magnetic field is filled with sulfur and oxygen ions being emitted by the volcanoes on the moon Io. At Saturn, the moon Enceladus jets water into space, filling Saturn’s magnetic field with water group ions.
• July 9, 2021: The 40-year-old mystery of what causes Jupiter’s X-ray auroras has been solved. For the first time, astronomers have seen the entire mechanism at work – and it could be a process occurring in many other parts of the Universe too. 20)
- Planetary astronomers have studied Jupiter’s spectacular X-ray auroral emission for decades. The X-ray ‘colors’ of these auroras show that they are triggered by electrically charged particles called ions crashing into Jupiter’s atmosphere. But astronomers had no idea how the ions were able to get to the atmosphere in the first place.
- Now, for the first time, they have seen the ions ‘surfing’ electromagnetic waves in Jupiter’s magnetic field, down into the atmosphere.
Figure 18: Jupiter’s mysterious X-ray auroras have been explained, ending a 40-year quest for an answer. For the first time, astronomers have seen the way Jupiter’s magnetic field is compressed, which heats the particles and directs them along the magnetic field lines down into the atmosphere of Jupiter, sparking the X-ray aurora. The connection was made by combining in-situ data from NASA’s Juno mission with X-ray observations from ESA’s XMM-Newton (image credit: Yao/Dunn/ESA/NASA)
Figure 19: Jupiter’s mysterious X-ray auroras explained (video credit: Yao/Dunn/ESA/NASA)
- The vital clues came from a new analysis of data from ESA’s XMM-Newton telescope and NASA’s Juno spacecraft. Situated in Earth’s orbit, XMM-Newton makes remote observations of Jupiter at X-ray wavelengths. Juno on the other hand circles the giant planet itself, taking in-situ readings from inside Jupiter’s magnetic field. But the question was: what should the team look for?
- The clue came when Zhonghua Yao, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, and lead author of the new study, realized that something didn’t make sense about Jupiter’s X-ray auroras.
- On Earth, auroras are visible only in a belt surrounding the magnetic poles, between 65 and 80 degrees latitude. Beyond 80 degrees, auroral emission disappears because the magnetic field lines here leave Earth and connect to the magnetic field in the solar wind, which is the constant flux of electrically charged particles ejected by the Sun. These are called open field lines and in the traditional picture, Jupiter and Saturn’s high-latitude polar regions are not expected to emit substantial auroras.
- However, Jupiter’s X-ray auroras are inconsistent with this picture. They exist poleward of the main auroral belt, pulsate regularly, and can sometimes be different at the north pole from the south pole. These are typical features of a ‘closed’ magnetic field, where the magnetic field line exits the planet at one pole and reconnects with the planet at the other.
- Using computer simulations, Zhonghua and colleagues previously found that the pulsating X-ray auroras could be linked to closed magnetic fields that are generated inside Jupiter and then stretch out millions of kilometers into space before turning back. 21)
- On 16 and 17 July 2017, XMM-Newton observed Jupiter continuously for 26 hours and saw X-ray auroras pulsating every 27 minutes. Simultaneously, Juno had been travelling between 62 and 68 Jupiter radii above the planet’s pre-dawn areas. This was exactly the area that the team’s simulations suggested were important for triggering the pulsations. So, the team searched the Juno data for any magnetic processes that were occurring at the same rate.
- They found that the pulsating X-ray auroras are caused by fluctuations of Jupiter’s magnetic field. As the planet rotates, it drags around its magnetic field. The magnetic field is struck directly by the particles of the solar wind and compressed. These compressions heat particles that are trapped in Jupiter’s magnetic field. This triggers a phenomenon called electromagnetic ion cyclotron (EMIC) waves, in which the particles are directed along the field lines.
- The particles themselves are electrically charged atoms called ions. Guided by the field, the ions ‘surf’ the EMIC wave across millions of kilometers of space, eventually slamming into the planet’s atmosphere and triggering the X-ray aurora.
- “What we see in the Juno data is this beautiful chain of events. We see the compression happen, we see the EMIC wave triggered, we see the ions, and then we see a pulse of ions traveling along the field line. And then a few minutes later, XMM sees a burst of X-rays,” says William Dunn, Mullard Space Science Laboratory, University College London, who co-led the research.
- Now that the process responsible for Jupiter’s X-ray auroras has been identified for the first time, it opens up a wealth of possibilities for where it could be studied next. For example, at Jupiter, the magnetic field is filled with sulphur and oxygen ions that are spewed out by the volcanoes on the moon Io. At Saturn, the moon Enceladus jets water into space, filling Saturn’s magnetic field with water ions.
- “This is a fundamental process that’s applicable to Saturn, Uranus, Neptune and probably exoplanets as well,” says Zhonghua.
- It may be more widely applicable even than that because now that the process has been revealed, there is a striking similarity to the ion auroras that happen here on Earth. In the case of Earth, the ion responsible is a proton, which comes from a hydrogen atom, and the process is not energetic enough to create X-rays. Yet, the basic process is that same. So, Jupiter’s X-ray aurora is fundamentally an ion aurora, although at much higher energy than the proton aurora on Earth.
- “It could be that EMIC waves play an important role in transferring energy from one place to another across the cosmos,” says William.
- As for Jupiter itself, the study of its auroras will continue with ESA’s JUpiter ICy moons Explorer (JUICE). Set to arrive by 2029, Juice will study the planet’s atmosphere, magnetosphere, and the effect that Jupiter’s four largest moons have on the auroras.
• June 29, 2021: New observations made with ESA’s X-ray XMM Newton telescope have revealed an “orphan cloud” – an isolated cloud in a galaxy cluster that is the first discovery of its kind. 22)
- A lot goes on in a galaxy cluster. There can be anything from tens to thousands of galaxies bound together by gravity. The galaxies themselves have a range of different properties, but typically contain systems with stars and planets, along with the material in between the stars – the interstellar medium. In between the galaxies is more material – tenuous hot gas known as the intercluster medium. And sometimes in all the chaos, some of the interstellar medium can get ripped out of a galaxy and get stranded in an isolated region of the cluster, as this new study reveals.
- Abell 1367, also known as the Leo Cluster, is a young cluster that contains around 70 galaxies and is located around 300 million light-years from Earth. In 2017, a small warm gas cloud of unknown origin was discovered in A1367 by the Subaru telescope in Japan. A follow-up X-ray survey to study other aspects of A1367 unexpectedly discovered X-rays emanating from this cloud, revealing that the cloud is actually bigger than the Milky Way.
Figure 20: This is the first time an intercluster clump has been observed in both X-rays and the light that comes from the warm gas. Since the orphan cloud is isolated and not associated with any galaxy, it has likely been floating in the space between galaxies for a long time, making its mere survival surprising (image credit: Chong Ge et al.)
- The discovery of this orphan cloud was made by Chong Ge at the University of Alabama in Huntsville, and colleagues, and the study has been published in Monthly Notices of the Royal Astronomical Society. 23)
- Along with data from XMM-Newton and Subaru, Chong and colleagues also used the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) to observe the cluster in visible light.
- The orphan cloud is the blue umbrella-shaped part of the image. It has been color-coded to show the X-ray part of the cloud in blue, the warm gas in red, and the visible region in white shows some of the galaxies in the cluster. The part of the cloud that had been discovered in 2017 (in red) overlaps with the X-ray at the bottom of the cloud.
How the cloud became an orphan
- It was previously thought that the distribution of material between galaxies is smooth, however more recent X-ray studies have revealed the presence of clumps in clusters. It was theorized that clumps of gas in the clusters were originally the gas that exists between stars in individual galaxies. The intercluster gas acts as a wind that is strong enough to pull the interstellar gas out of the galaxy as the galaxy is moving through the cluster. However, observations showing that intercluster clumps are originally stripped interstellar material have never been made until now. The observation of the warm gas in the clump provides the evidence to show that this orphan cloud originated within a galaxy. Interstellar material is much cooler than intercluster material, and the temperature of the orphan cloud matches that of interstellar gas. The researchers were also able to determine why the orphan cloud has survived for as long as it has. An isolated cloud would be expected to be ripped apart by instabilities caused by velocity and density differences. However, they found that a magnetic field in the cloud would be able to suppress these instabilities.
Searching for the parent galaxy
- It is likely that the parent galaxy of the orphan cloud is a massive one as the mass of the X-ray gas in the orphan is substantial. It is possible that the parent might one day be discovered with future observations by following some breadcrumbs. For example, there are traces of the warm gas that extend beyond the orphan cloud that could be used to identify the parent with more data. There are other unsolved mysteries regarding the cloud that could be deciphered with more observations, such as mysterious offset between the brightest X-rays and the brightest light from the warm gas.
- A closer inspection of this orphan will also further our understanding of the evolution of stripped interstellar medium at such a great distance from its parent galaxy and will provide a rare laboratory to study other things such as turbulence and heat conduction. This study paves the way for research on intercluster clumps, as future warm gas surveys can now be targeted to search for other orphan clouds.
• January 15, 2021: A new study, led by a theoretical physicist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), suggests that never-before-observed particles called axions may be the source of unexplained, high-energy X-ray emissions surrounding a group of neutron stars. 24)
- First theorized in the 1970s as part of a solution to a fundamental particle physics problem, axions are expected to be produced at the core of stars, and to convert into particles of light, called photons, in the presence of a magnetic field.
- Axions may also make up dark matter – the mysterious stuff that accounts for an estimated 85 percent of the total mass of the universe, yet we have so far only seen its gravitational effects on ordinary matter. Even if the X-ray excess turns out not to be axions or dark matter, it could still reveal new physics.
- A collection of neutron stars, known as the Magnificent 7, provided an excellent test bed for the possible presence of axions, as these stars possess powerful magnetic fields, are relatively nearby – within hundreds of light-years – and were only expected to produce low-energy X-rays and ultraviolet light.
- “They are known to be very ‘boring,’” and in this case it’s a good thing, said Benjamin Safdi, a Divisional Fellow in the Berkeley Lab Physics Division theory group who led a study, published Jan. 12 in the journal Physical Review Letters, detailing the axion explanation for the excess. Christopher Dessert, a Berkeley Lab Physics Division affiliate, contributed heavily to the study, which also had participation by researchers at UC Berkeley, the University of Michigan, Princeton University, and the University of Minnesota. 25)
- If the neutron stars were of a type known as pulsars, they would have an active surface giving off radiation at different wavelengths. This radiation would show up across the electromagnetic spectrum, Safdi noted, and could drown out this X-ray signature that the researchers had found, or would produce radio-frequency signals. But the Magnificent 7 are not pulsars, and no such radio signal was detected. Other common astrophysical explanations don’t seem to hold up to the observations either, Safdi said.
- If the X-ray excess detected around the Magnificent 7 is generated from an object or objects hiding out behind the neutron stars, that likely would have shown up in the datasets that researchers are using from two space satellites: the European Space Agency’s XMM-Newton and NASA’s Chandra X-ray telescopes.
- Safdi and collaborators say it’s still quite possible that a new, non-axion explanation arises to account for the observed X-ray excess, though they remain hopeful that such an explanation will lie outside of the Standard Model of particle physics, and that new ground- and space-based experiments will confirm the origin of the high-energy X-ray signal.
- “We are pretty confident this excess exists, and very confident there’s something new among this excess,” Safdi said. “If we were 100% sure that what we are seeing is a new particle, that would be huge. That would be revolutionary in physics.” Even if the discovery turns out not to be associated with a new particle or dark matter, he said, “It would tell us so much more about our universe, and there would be a lot to learn.”
- Raymond Co, a University of Minnesota postdoctoral researcher who collaborated in the study, said, “We’re not claiming that we’ve made the discovery of the axion yet, but we’re saying that the extra X-ray photons can be explained by axions. It is an exciting discovery of the excess in the X-ray photons, and it’s an exciting possibility that’s already consistent with our interpretation of axions.”
- If axions exist, they would be expected to behave much like neutrinos in a star, as both would have very slight masses and interact only very rarely and weakly with other matter. They could be produced in abundance in the interior of stars. Uncharged particles called neutrons move around within neutron stars, occasionally interacting by scattering off of one another and releasing a neutrino or possibly an axion. The neutrino-emitting process is the dominant way that neutron stars cool over time.
Figure 21: An artistic rendering of the XMM-Newton (X-ray Multi-Mirror Mission) space telescope. A study of archival data from the XMM-Newton and the Chandra X-ray space telescopes found evidence of high levels of X-ray emission from the nearby Magnificent Seven neutron stars, which may arise from the hypothetical particles known as axions [image credits: D. Ducros, ESA/XMM-Newton, CC BY-SA 3.0 IGO)]
- Like neutrinos, the axions would be able to travel outside of the star. The incredibly strong magnetic field surrounding the Magnificent 7 stars – billions of times stronger than magnetic fields that can be produced on Earth – could cause exiting axions to convert into light.
- Neutron stars are incredibly exotic objects, and Safdi noted that a lot of modeling, data analysis, and theoretical work went into the latest study. Researchers have heavily used a bank of supercomputers known as the Lawrencium Cluster at Berkeley Lab in the latest work.
- Some of this work had been conducted at the University of Michigan, where Safdi previously worked. “Without the high-performance supercomputing work at Michigan and Berkeley, none of this would have been possible,” he said. “There is a lot of data processing and data analysis that went into this. You have to model the interior of a neutron star in order to predict how many axions should be produced inside of that star.”
- Safdi noted that as a next step in this research, white dwarf stars would be a prime place to search for axions because they also have very strong magnetic fields, and are expected to be “X-ray-free environments.”
- “This starts to be pretty compelling that this is something beyond the Standard Model if we see an X-ray excess there, too,” he said.
- Researchers could also enlist another X-ray space telescope, called NuStar, to help solve the X-ray excess mystery.
- Safdi said he is also excited about ground-based experiments such as CAST at CERN, which operates as a solar telescope to detect axions converted into X-rays by a strong magnet, and ALPS II in Germany, which would use a powerful magnetic field to cause axions to transform into particles of light on one side of a barrier as laser light strikes the other side of the barrier.
- Axions have received more attention as a succession of experiments has failed to turn up signs of the WIMP (weakly interacting massive particle), another promising dark matter candidate. And the axion picture is not so straightforward – it could actually be a family album.
- There could be hundreds of ALPs (Axion-Like Particles) that make up dark matter, and string theory – a candidate theory for describing the forces of the universe – holds open the possible existence of many types of ALPs.
- The study was supported by the U.S. Department of Energy Office of Science Early Career Research Program; Advanced Research Computing and the Leinweber Graduate Fellowship at the University of Michigan, Ann Arbor; the National Science Foundation; the Mainz Institute for Theoretical Physics (MITP) of the Cluster of Excellence PRISMA+; the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG Excellence Cluster Origins; and the CERN Theory department.
• January 11, 2021: A team of astronomers led by Lidia Oskinova of the University of Potsdam, Germany, used ESA’s XMM-Newton X-ray telescope to study the object that was originally discovered in 2019. Back then, astronomers already reported that the object has very high wind speeds and is too bright, and therefore too massive, to be an ordinary white dwarf. They suggested that the object is a new type of star that survived the merger of two white dwarfs. 26)
- Based on new information from XMM-Newton, Lidia and her team now suggest that what we see in the image is a new type of X-ray source powered by the merger of two white dwarfs. The remnant of the clash – the nebula – is also visible in this image, and is mostly made out of the element neon (shown in green). The star is very unstable and will likely collapse into a neutron star within 10,000 years. 27)
Figure 22: This image shows a new type of star that has never been seen before in X-ray light. This strange star formed after two white dwarfs – remnants of stars like our Sun – collided and merged. But instead of destroying each other in the event, the white dwarfs formed a new object that shines bright in X-ray light (ESA/XMM-Newton, L. Oskinova/Univ. Potsdam, Germany)
• November 12, 2020: This burst of color shows a fascinating discovery: a galaxy cluster acting as a cosmic furnace. The cluster is heating the material within to hundreds of millions of degrees Celsius – well over 25 times hotter than the core of the Sun. 28)
- The cluster, named HSC J023336-053022 (XLSSC 105), lies four billion light-years from Earth and was independently discovered by both ESA’s spaceborne XMM-Newton X-ray Observatory and NAOJ’s Subaru optical-infrared telescope in Hawaii, USA. XMM-Newton detected the cluster via the international XXL survey, which is exploring two large areas of space outside our galaxy.
- Galaxies are not distributed randomly throughout the Universe, and instead exist within groups and larger clusters. These aggregations can be mammoth and sometimes contain many thousands of individual galaxies in a single structure, all embedded in clumps of invisible dark matter. Different sub-groups of galaxies can also form within a single cluster, as shown here by the two blue-purple circles on either side of centre. These circles mark the locations of two sub-clusters within HSC J023336-053022 which are slowly moving towards and colliding with one another, ‘shock heating’ gas to intense temperatures in the process.
- The addition of radio observations makes this image special, as many studies of collisions within or between galaxy clusters have not captured this shock-heating process – which is represented visually in the region where green changes to red – in radio. This process releases immense amounts of energy and heats already scorching gas to temperatures tens of times hotter. Before shock heating, the gas sits at around 40 million degrees Celsius – already some 2.7 times hotter than the core of the Sun.
Figure 23: To create this image, three different international teams of astronomers explored observations of the cluster across the electromagnetic spectrum, in order to isolate and pinpoint different aspects of this region of space. These aspects are shown here in different colors. Individual galaxies within the cluster show up in orange, and dark matter – which maps the location of the two sub-clusters – in blue (via optical observations from Subaru). Hot, dense gas shows up in green (X-ray from XMM-Newton), while hot, thin, high-pressure gas shows up in red (radio from the Green Bank Telescope in Virginia, USA). This gas is something known as the ‘intracluster medium’, which permeates galaxy clusters and fills the space between galaxies [image credit: Radio: GBT Green Bank Observatory/National Science Foundation (NSF); Optical: Subaru Telescope, National Astronomical Observatory of Japan/HSC-SSP collaboration; X-ray: European Space Agency (ESA)/XMM-Newton/XXL survey consortium]