eROSITA Advent Calendar

eROSITA is the primary instrument on-board the Russian-German "Spectrum-Roentgen-Gamma" (SRG) mission, which was successfully launched from Baikonur on July 13, 2019 and placed in a halo orbit around the L2 point. In 2020, eROSITA has perfomed its first all-sky survey (and is close to finishing the second scan).

In this advent calender, on the backdrop of the first all-sky survey image, you'll find each day a new highlight from the first year of operation of eROSITA as well as some background information on the telescope itself. Enjoy!
_{(c) IKI/MPE}

Navigating the X-ray sky

Over the course of 182 days, the eROSITA X-ray telescope onboard SRG completed its first full sweep of the sky in June 2020. This new map of the hot, energetic universe contains more than one million objects, roughly doubling the number of known X-ray sources discovered over the 60-year history of X-ray astronomy. Most of the new sources are active galactic nuclei at cosmological distances, marking the growth of gigantic black holes over cosmic time. Clusters of galaxies in the new map will be used to track the growth of cosmic structures and constrain cosmological parameters. Closer to home, stars with hot coronae, binaries and supernova remnants dot our Galaxy, and we now have a complete map of the hot baryons in the Milky Way, something that can only be achieved with the 360-degree view provided by the eROSITA survey.

The Shapley Supercluster seen in the eROSITA all-sky survey

The Shapley supercluster of galaxies is one of the most massive concentrations of galaxies in the local universe at a distance of about 650 million light-years (z~0.05). Each of the dozen extended structures is itself a cluster of galaxies, consisting of 100s to 1000s of individual galaxies, each denoting an intersection of filaments making up the large-scale structure in the Universe. This image spans 16 degrees across the sky (about 30 times the size of the full moon), which translates into about 180 million light-years across at the distance of the Shapley supercluster. The images on the left show a zoom of the the most massive clusters in the Shapley supercluster.

eROSITA's sparkling sky: the particle background

When eROSITA's eyes are closed, scintillating events can still be seen. They are caused by high-energy particles, such as Galactic cosmic rays or energetic solar particles. These can penetrate the shielding made of 3 centimetres of copper, and hit the sensitive area of the pnCCD camera. In addition, these particles can also interact with the camera housing and create a variety of secondary particles, forming showers of events that are accompanied by X-ray emission.

Usually, these events are rejected on-board, to save telemetry for the actual eROSITA science. On special occasions and for a short amount of time, full images with all events are beamed to ground, to see the full glory of space environment at the Lagrange point L2. The animation shows 140 such images, with total exposure of only 7 seconds, taken on the evening of 1st April 2020.

The Sun's 11-year activity cycle modulates the Galactic Cosmic Rays, and at the current solar minium the protecting magnetic fields are weakest. eROSITA could recently see a decline of this particle background, expected for the start of the new solar cycle. But also variations on the order of 27 days are visible, maybe coupled to solar rotation.
M. Freyberg et al., "SRG/eROSITA in-flight background at L2", Proc. SPIE 11444, https://doi.org/10.1117/12.2562709

Operating eROSITA on-board SRG

SRG mission control is carried out from Moscow. Spacecraft control, flight dynamics and ground antenna interfacing take place at the Mission Control Center (MCC) of the Lavochkin Research and Production Association. There is no continuous communication with the spacecraft: ground contact passes are planned daily with an average of 4 hours per day. The main ground stations involved in operations are large (~70 meters) dishes located in Bear Lakes, near Moscow, and in Ussuriysk, in the Russian far east.

During these 4 hours, MPE is in direct contact with eROSITA and control and operations takes place in the eROSITA control room at the institute. The operators-on-duty receive real time data, which is analysed and checked to verify the integrity of all the various eROSITA subsystems. Commands are executed in real-time. Real-time photons are received and displayed for a quick-view analysis before they are stored for scientific analysis. Before such a photon is displayed in one of the control room monitors, it travels 1.5 million kilometres from the location of the spacecraft to Earth and then crosses the whole of the Eurasian continent, from Ussuriysk to Munich. That is, of course, insignificant compared to the actual distance the photon has travelled before being recorded with the eROSITA instruments...

The eROSITA cameras

The eROSITA X-ray telescope is equipped with an array of seven cameras – one for each of the seven mirror systems. Based on the EPIC-pnCCD detector, the custom-made pnCCD detectors for eROSITA were developed at MPE, taking into account lessons learned during the XMM-Newton project.

The eROSITA detectors enable spectroscopic, time and spatially resolved observations with high quantum efficiency in the 0.2–10 keV energy range. The performance has been significantly improved compared to the XMM-Newton pnCCD detector. This allows eROSITA to perform spectroscopy also at low energies, e.g. it can resolve the K-line of carbon at 277 eV.

Since 15^th of October 2019 all seven cameras observe the X-ray sky with a performance similar to that of the ground calibration measurements. The image shows the eROSITA focal plane camera array with one camera in the center and six cameras surrounding it.

A detailed description of the eROSITA cameras can be found in this publication:

The eROSITA Camera Array on the SRG Satellite, Norbert Meidinger et al., SPIE Proc. 11444, to be published

eROSITA Mirrors

eROSITA uses a very special kind of telescope, which can focus X-rays! This would not be possible with ordinary telescopes, because they would essentially just absorb them. However, if X-rays hit an extremely flat surface at a very small angle, they get reflected with sufficient efficiency. This makes a focusing X-ray 'mirror' appear cylindrical. Its surface, however, is more complicated: it needs to be composed of a paraboloid and a hyperboloid shell. When looking at such a mirror module from a large distance, all we see from such a surface is a narrow ring, and only this ring is collecting the X-rays. In order to achieve a larger collection area, the interior of the ring can be filled with another mirror module, and yet another one... Each eROSITA mirror assembly is equipped with 54 of them! And this is not all: eROSITA carries seven such mirror assemblies, each composed of 54 mirror shells! They all look into the same direction, and each mirror assembly gets an X-ray detector of its own, to form one telescope module. Finally, all the seven telescope modules together make the eROSITA telescope.

The eFEDS Supercluster

Superclusters of galaxies are among the largest structures in the Universe. They contain tens of galaxy clusters, with each galaxy cluster containing hundreds of galaxies, with each galaxy containing a thousand billion stars like our Sun. In the eFEDS mini-survey carried out during the Performance Verification phase, eROSITA detected about 5 such superclusters. The image shows the one which is the furthest away from us; the X-ray photons detected by eROSITA took 4 billion years to travel until being recorded by the telescope.
In this supercluster, three galaxy clusters are in the process of merging, which are shown in the enlarged region as an optical image. The large shocks induced by the merger in the intracluster medium shine in the radio waveband, denoted by the white contours. These observations allow us to study the physical processes that take place during structure formation at a node of the cosmic web, where a huge amount of matter is gravitationally pulled in.

A young quasar

In the eFEDS field, which was imaged by eROSITA in its commissioning phase, astronomers have identified a well-known high-redshift quasar: SDSS J083643.85+005453.3 . With a redshift of z=5.81, it is one of only two X-ray selected sources at such a large distance known to date. It is also one of the most X-ray luminous high-redshift quasars. The multi-wavelength properties of this quasar show characteristics that are typical for young jets. As the light form this source was emitted when the Universe was only 1 billion years old, the study of the quasar in a contiguous field allowed the team to investigate the AGN demographics at the end of the epoch of reionization. First constraints on the X-ray luminosity function at this cosmic time will give further insights into the space density of accreting super-massive black holes at high redshifts, providing new information on the formation and initial growth mechanisms of these objects. The "detection plot" shows the eROSITA image and a zoomed-in image from the HSC survey.

Vela and Friends

Due to its size and close distance to Earth, the "Vela supernova remnant" which is shown in this picture is one of the most prominent objects in the X-ray sky. The Vela supernova exploded about 12000 years ago at a distance of 800 light-years and overlaps with at least two other supernova remnants, Vela Junior (in the picture seen as bluish ring at the bottom left) and Puppis-A (top right). Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this explosion were found in polar ice cores. All three supernova explosions produced both the X-ray-bright supernova remnants and neutron stars, which shine as intense X-ray point sources near the centres of the remnants. The quality of the new eROSITA data of this "stellar cemetery" will give astronomers many exciting new insights into the physical processes operating in the hot supernova plasma as well as for exploring the exotic neutron stars.

Calibrating eROSITA

The complicated structure of the eROSITA optics with 7 nested mirrors (see entry for 6 December), made necessary by the properties of X-rays, has its price: the image quality degrades noticably from the center to the outer areas of the field of view, where objects are getting fuzzier and fainter. And the image quality depends on the energy (the 'color') of the X-rays. In order to be able to analyse X-ray images, especially when they drift across the field of view, as during the eROSITA sky surveys, it is necessary to know all the artefacts which are introduced by the X-ray optics or, in other words, the optics needs to be calibrated. This is a very challenging task, especially because this must be done in a vacuum, with the X-ray source placed far away from the optics. The Figure shows one result of the calibration: we see how a point-like X-ray source would look like when placed at various positions in the field of view. In order to obtain this information, 726 measurements were necessary (121 positions at 6 energies).

A weird, large amplitude X-ray flare in the nucleus of a galaxy

eROSITA’s All Sky Survey will scan the whole sky every 6 months. This will allow astronomers to monitor how the X-ray emission from supermassive black holes changes over a four year period. On 28th April 2020, only a few months into its first all sky survey, eROSITA detected a large flaring in the X-ray emission from a galaxy that had shown no prior signs of hosting an actively accreting black hole. This suggests that the black hole had been starved of fuel and an explosive event must have happened recently to trigger a vast increase in the accretion rate. The plot here shows how the brightness of the source AT 2019avd varies over time (from October 2018 to September 2020), with the top panel showing the mid-infrared flaring observed by NEOWISE, the middle showing the optical emission in two different energy bands, as observed by the Zwicky Transient Facility, and the bottom plot showing the X-ray measurements from the first eROSITA survey (eRASS1) and from the XRT instrument on the Swift satellite as well as the previous upper limit from XMM-Newton. Whilst the observed X-ray flare could be produced in the aftermath of a star being tidally shredded by a black hole, the optical and ultraviolet lightcurves of the event look completely different to other known tidal disruption events. Furthermore, the observed properties of the event do not match well any known cases of black hole variability to date. The physical mechanism responsible for producing such drastic changes in the emission from the system is still not clear.

ROSAT - The father of X-ray surveys

15 years ago, eROSITA was "ROSAT's little daughter" - today, this little daughter has fully grown up.

ROSAT was built in the 1980s under the scientific direction of MPE and launched into space in 1990 on a Delta-II rocket. It was a mission with international participation: The UK provided an EUV telescope and the wide field camera; one of the three detectors for the main X-ray telescope, the High Resolution Imager (HRI), came from NASA, who was also responsible for launch. With a resolution of better than 5 arcseconds, ROSAT was unprecedented at the time, as was its field of view of 2 degrees in diameter. In only half a year, the X-ray telescope performed a survey of the complete sky, but in these six months 100,000 X-ray sources were discovered and X-ray astronomy grew by a factor of about 20. And although the mission time has been planned for just 1.5 years, ROSAT was operated until 1998, making the mission a huge success. In fact, ROSAT could be called the father of X-ray imaging surveys.

eROSITA finds large-scale bubbles in the halo of the Milky Way

The first all-sky survey performed by the eROSITA X-ray telescope on-board the Spektrum-Roentgen-Gamma (SRG) observatory has revealed a large hourglass-shaped structure in the Milky Way. These “eROSITA bubbles” show a striking similarity to the Fermi bubbles, detected a decade ago at even higher energies. The most likely explanation for these huge features is a massive energy injection from the Galactic centre in the past, leading to shocks in the hot gas envelope around our galaxy.

SRG/eROSITA Launch

At 14:31 on 13 July 2019, the Spektrum-Roentgen-Gamma (SRG) space mission successfully lifted off from the Baikonur cosmodrome. On-board was the eROSITA X-ray telescope, which was developed and built by a consortium of German institutes supported by DLR and led by the Max Planck Institute for Extraterrestrial Physics (MPE). Now in operation along a large L2 orbit, 1.5 million kilometres from Earth, eROSITA will perform a deep survey of the entire X-ray sky over the next four years, providing the first ever deep imaging survey of the sky at soft and hard X-rays. In total, eROSITA will scan the sky 8 times, with the first two all-sky surveys done in 2020.

A giant dust scattering ring

In February 2020, eROSITA discovered this glowing ring when scanning over the southern Milky Way. The ring is caused by scattered X-ray radiation from the faint blue object in the centre of the ring. This object, likely a black hole circled by a companion star, underwent a massive outburst one year before the eROSITA observation. On its thousands years travel a tiny fraction of the burst radiation was scattered by a dust cloud in a spiral arm of the Milky Way. Due to this detour through our galaxy the scattered X-rays arrive later than the direct radiation from the burst and form a faint ring growing with time.
With a diameter of 1.3 degrees this is by far the largest X-ray scattering ring discovered to date. By combining the eROSITA image of the ring with data from the ESA satellites XMM-Newton and Gaia an accurate distance of 3390 pc (11000 light-years) to the black hole binary was measured.
Reference: G. Lamer G., A. Schwope, P. Predehl, I. Traulsen, J. Wilms, M. Freyberg, 2020, A&A, accepted, http://arxiv.org/abs/2012.11754

Study for Dark Matter and Dark Energy funded by ERC

An ERC consolidator grant will enable Dr. Esra Bulbul and her working group to embark on the project “DarkQuest: Shedding Light on the Nature of Dark Matter and Dark Energy with Multi-Wavelength All-Sky Surveys.” Taking advantage of the most recent eROSITA survey, the cluster science and cosmology working group will use galaxy clusters as a probe to study the cosmological parameters. Galaxy clusters are massive objects, dominated by dark matter. Sitting at the nodes of the cosmic web, they provide a representative view of the distribution of dark matter, the most abundant matter in the Universe. Their abundance as a function of redshift and mass is a sensitive probe of the growth of structure in the Universe, and hence can be used to constrain dark energy. The eFEDS mini survey, carried out during the eROSITA Performance Verification phase, already detected 540 extended sources.

Introducing eROSITA on SRG

The eROSITA X-ray telescope is the main scientific payload on the Russian-German Astrophysics space mission Spektrum-Roentgen-Gamma (SRG). It was designed and built, over a period of about 10 years, by a consortium of German Institutes (and industrial partners all over the world) led by MPE. Launched from Baykonur in July 2019, it has been taking scientific data since October 2019. Peter Predehl, Principal Investigator of eROSITA throughout its development and early operation phases, and his collaborators introduce eROSITA in this publication, which provides a concise description of the instrument, its performance as measured on ground, its operation in space and also of the first results from in-orbit measurements.
The image is a schematic view of the eROSITA telescope with (from top): Front cover, 7 Mirror Assemblies (baffle, mirror, electron deflector) and 7 Camera Assemblies (filter wheel, camera, electronics box). In addition there are two startrackers (one seen) and four large radiators to cool the cameras and their electronics. The telescope structure is lightly shaded.
Reference: Predehl et al. 2020 (A&A, in press); arXiv:2010.03477

SRG/eROSITA completes its second all-sky survey

Six more months of operations, a second X-ray scan of the entire sky is completed! This new all-sky picture will be used, in combination with the first one completed in June, by scientists in Germany and Russia to explore in greater detail the X-ray sky. Fainter and more distant objects can be detected in the combied map. In addition, a comparison of the images taken six months apart reveal a plethora of variable and transient sources dotting the dynamical X-ray sky: flaring stars, exploding stars, accreting black holes and neutron stars, and many others. The picture is an 'exposure map' of the whole sky in equal-area 'Aitoff' projection. It shows for how long each point of the sky was observed by eROSITA during one all-sky survey; black are most exposed areas (more than twnety minutes), while red are the least exposed areas (less than 100 seconds). The figure refers to eRASS1, but each and every survey has a similar exposure distribution, mainly dictated by the geometry of the SRG orbit.

Revealing the Beauty of the Hidden Universe: eROSITA sees first light

On 22 October 2019, the beautiful first X-ray images of the eROSITA telescope on SRG were presented to the public. After an extended commissioning phase, since 13 October 2019 all seven X-ray telescope modules with their custom-designed CCD cameras started observing the sky simultaneously. The first combined X-ray images of our neighbouring galaxy, the Large Magellanic Cloud, showed remarkable details and demonstrate the promise of the ambitious science programme planned with the space-borne telescope.

A supermassive black hole dims its X-ray lights

One of the most prominent AGNs, the ultra-soft Narrow-Line Seyfert 1 Galaxy 1H0707-495, has been observed with eROSITA as one of the first CalPV observations on October 13, 2019 for about 60.000 seconds. 1H 0707-495 is a highly variable AGN, with a complex, steep X-ray spectrum, which has been the subject of intense study with XMM-Newton in the past. The eROSITA light curves show significant variability in the form of a flux decrease by a factor of about 100. The change in luminosity corresponds to the “switch off” of about 1 Billion solar luminosities in about one day. eROSITA photons are ultra-soft and most of the variability is seen below 0.8 keV. This behavior has not been seen in other AGN before and has been detected with eROSITA for the first time. An ionized partial coverer and strong relativistic reflection explains the unique X-ray softness.
Reference: Th. Boller et. al, A&A accepted, https://doi.org/10.1051/0004-6361/202039316

The Dark Universe

The accelerating expansion of the Universe represents one of the greatest discoveries of modern physics. In the current cosmological model, the dark energy component dominates the content of the Universe with 69 percent, which is responsible for its acceleration. The majority of the remainder is in mysterious dark matter (26 percent), while the baryons constitute only about 5 percent. Clusters of galaxies representing the largest peaks in the matter density field, their distribution as a function of mass and redshift is highly sensitive to the matter density and the growth of structure. In its search for clues about dark matter and dark energy, eROSITA will study the distribution of approximately 100,000 galaxy clusters in the Universe.

The supernova remnant N132D in the LMC

The supernova remnant N132D in the Large Magellanic Cloud is at a distance of about 160000 light-years, the corresponding supernova explosion happened about 3000 years ago. The eROSITA observations not only demonstrate the extremely high dynamical range (the surface brightness of N132D is 5000 times higher than its surroundings) and excellent spatial (top right) but also spectral resolution (bottom right). In the eROSITA spectrum, i.e. the „X-ray rainbow“ lines from several heavy elements can be idenfied: oxygen (O), iron (Fe), neon (Ne), magnesium (Mg), and silicon (Si).

A supergiant fast X-ray transient in the LMC

During the Calibration and Performance-verification phase, eROSITA observed an an apparently typical high-mass X-ray binary in the Large Magellanic Cloud (XMMU J053108.3−690923) and caught some fast flares. This may be the first conclusive evidence of a supergiant fast X-ray transient (SFXT) outside our Galaxy. Composed of a supergiant luminous star and a compact object such as a neutron star, SFXTs derive their name from their strong and extremely fast-rising X-ray flares that usually last for a few-hours.

In most of the known SFXTs, flares are sporadic. The long exposure and unprecedented sensitivity offered by eROSITA have provided a unique opportunity to uncover the extreme variable nature of the source for the first time. The picture shows that the X-ray intensity exhibited multiple fast flares lasting for a few hours each. The total range of variability by comparison with earlier results spanned more than three-orders of magnitude and qualifies XMMU J053108.3−690923 as a textbook example of an SFXT.

During the course of its eight all-sky surveys, the eROSITA observations will allow a near continuous monitoring of high-mass X-ray binaries in the Magellanic Clouds. This will uncover many more such SFXTs whose true nature is waiting to be revealed.
The paper describing this result can be found here: https://doi.org/10.1051/0004-6361/202039468

Longest intergalactic gas filament discovered

More than half of the matter in our universe has so far remained hidden from us. However, astrophysicists had a hunch where it might be: In so-called filaments, unfathomably large thread-like structures of hot gas that surround and connect galaxies and galaxy clusters. eROSITA has now for the first time observed a gas filament with a length of 50 million light years. Its structure is strikingly similar to the predictions of computer simulations. The observation therefore also confirms our ideas about the origin and evolution of our universe. The still image is taken from a simulation showing the distribution of hot gas (left), compared with the eROSITA X-ray image of the Abell 3391/95 system (right).