11institutetext: Space Research Institute (IKI), Russian Academy of Sciences,
Profsoyuznaya ul. 84/32, Moscow, 117997 Russia 22institutetext: Max-Planck-
Institut für Astrophysik (MPA), Karl-Schwarzschild-Str. 1, D-85741 Garching,
Germany 33institutetext: Lavochkin Association, 24 Leningradskaya ul., Khimki
141400, Moscow Region, Russia 44institutetext: State Space Corporation
Roscosmos, Schepkina ulitsa 42, Moscow, 107996, Russia 55institutetext: Max-
Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstr. D-85748
Garching, Germany 66institutetext: ESAC Camino bajo de Casillo S/N, Villanueva
de la Canada, Madrid, Spain 77institutetext: Deutsches Zentrum fürr Luft- und
Raumfahrt, Königswinterer Str. 522-524, D-53227 Bonn, Germany 88institutetext:
Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg

# SRG X-ray orbital observatory. Its telescopes and first scientific results

R. Sunyaev SRG X-ray orbital observatory. Its telescopes and first scientific
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Hasinger SRG X-ray orbital observatory. Its telescopes and first scientific
resultsSRG X-ray orbital observatory. Its telescopes and first scientific
results I. Khabibullin SRG X-ray orbital observatory. Its telescopes and first
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scientific resultsSRG X-ray orbital observatory. Its telescopes and first
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scientific results

The orbital observatory Spectrum-Roentgen-Gamma (SRG), equipped with the
grazing-incidence X-ray telescopes Mikhail Pavlinsky ART-XC and eROSITA, was
launched by Roscosmos to the Lagrange L2 point of the Sun–Earth system on July
13, 2019. The launch was carried out from the Baikonur Cosmodrome by a
Proton-M rocket with a DM-03 upper stage. The German telescope eROSITA was
installed on SRG under an agreement between Roskosmos and the DLR, the German
Aerospace Agency. In December 2019, SRG started to perform its main scientific
task: scanning the celestial sphere to obtain X-ray maps of the entire sky in
several energy ranges (from 0.2 to 8 keV with eROSITA, and from 4 to 30 keV
with ART-XC). By mid-June 2021, the third six-month all-sky survey had been
completed. Over a period of four years, it is planned to obtain eight
independent maps of the entire sky in each of the energy ranges. The sum of
these maps will provide high sensitivity and reveal more than three million
quasars and over one hundred thousand massive galaxy clusters and galaxy
groups. The availability of eight sky maps will enable monitoring of long-term
variability (every six months) of a huge number of extragalactic and Galactic
X-ray sources, including hundreds of thousands of stars with hot coronae. In
addition, the rotation of the satellite around the axis directed toward the
Sun with a period of four hours enables tracking the faster variability of
bright X-ray sources during one day every half year. The chosen strategy of
scanning the sky leads to the formation of deep survey zones near both
ecliptic poles. The paper presents sky maps obtained by the telescopes on
board SRG during the first survey of the entire sky and a number of results of
deep observations performed during the flight to the L2 point in the frame of
the performance verification program, demonstrating the capabilities of the
observatory in imaging, spectroscopy, and timing of X-ray sources. It is
planned that in December 2023, the observatory will for at least two years
switch to observations of the most interesting sources in the sky in triaxial
orientation mode and deep scanning of selected celestial fields with an area
of up to 150 square degrees. These modes of operation were tested during the
performance verification phase. Every day, data from the SRG observatory are
dumped onto the largest antennas of the Russian Deep Space Network in Bear
Lakes and near Ussuriysk.

###### Key Words.:

Space vehicles: instruments – X-rays: general – Surveys

##  1 Introduction

The Spectrum-Roentgen-Gamma (SRG) orbital observatory111http://srg.cosmos.ru
was launched into a halo orbit around the Lagrange L2 point of the Sun–Earth
system on July 13, 2019, from the Baikonur Cosmodrome by a Russian Proton
rocket with a DM-03 booster (Fig. 1). The Navigator platform (total mass 2712
kg), developed by NPO Lavochkin (NPOL) in Khimki near Moscow, carries a
scientific payload (total mass 1170 kg) consisting of two X-ray telescopes
with grazing-incidence optics (Figs. 2 and 3): extended Roentgen Survey with
an Imaging Telescope Array (eROSITA, Predehl et al. 2021), developed in the
Max Planck Institute for extraterrestrial Physics (MPE), Germany, and the
Mikhail Pavlinsky ART-XC (Astronomical Roentgen Telescope – X-ray
Concentrator), developed in Russia (Pavlinsky et al. 2021b). This scheme is
implemented in accordance with a memorandum signed in 2007 between the Russian
Space Agency (at that time) Roscosmos and the German Aerospace Agency,
Deutsches Zentrum für Luft- und Raumfahrt (DLR). The eROSITA telescope is
sensitive to X-rays in the energy range from 200 eV to 8 keV, and ART-XC is
sensitive at 4–30 keV.

The Navigator platform has been developed as a universal medium-class platform
for scientific and meteorological missions to be launched into various orbits.
Since January 2011, the Navigator platform has been used in the three Elekro-L
meteorological satellite missions, as well as in the scientific Spektr-R
mission (RadioAstron, Kardashev et al. 2013), which was launched in 2011 and
operated until 2018.

Figure 1: Baikonur launch site (Kazakhstan): Proton rocket and the DM-03 upper
stage with the SRG spacecraft. Figure 2: SRG orbital observatory with the
folded solar panels in NPO Lavochkin’s assembly hall before shipment to
Baikonur. Figure 3: SRG observatory in flight (artist’s impression). Each
X-ray telescope consists of seven independent mirror modules. Figure 4:
Russian Deep Space Network for the SRG mission.

The SRG spacecraft is operated by NPOL’s Control Center, while the
responsibility for operation of the X-ray telescopes lies with the MPE in
Garching near Munich (eROSITA) and the Space Research Institute (IKI) of the
Russian Academy of Sciences in Moscow (ART-XC). The downlink and uplink of
spacecraft telemetry and commanding data is performed by the Centers of Deep
Space Communications in Bear Lakes (64 m diameter antenna) near Moscow and
Ussuriysk (70 m diameter antenna) in the Russian Far East as well as Baikonur
(12 m antenna) placed in Kazakhstan Fig. 4.

The daily command uplink and scientific data downlink take about one hour for
ART-XC and from 2.5 to 4 hours for eROSITA (including health checks of the
telescopes’ detectors and subsystems). The uplink of commands to the
spacecraft and telescopes is performed using the antennas mentioned above or
additional 12m diameter antennas in Baikonur and Bear Lakes.

The SRG orbit allows observations to be conducted around the clock with just
short breaks for uploading and implementation of operating commands for the
Navigator platform and the telescopes. The Navigator platform and both
telescopes have onboard mass memory, allowing them to store the data
accumulated over several days and dump them to the receiving stations during
daily communication sessions. During downlink, the telescopes can (in most
cases) continue observations and accumulation of data.

The eROSITA and ART-XC telescopes each have seven independent mirror systems
and seven independent positionally sensitive detectors in their focal planes
(see Fig. 39 for ART-XC and Fig. 42 for eROSITA), six of which are coaligned
on a hexagon surrounding an identical central mirror system and detector. To
focus X-rays, both telescopes use grazing incidence optics (Wolter 1952a, b).
Figure 40 demonstrates the Wolter I optical scheme of each of the seven ART-XC
telescope modules, consisting of 28 pairs of coaxial parabolic and hyperbolic
mirror shells, and a detector at the focus of each.

During the 100-day flight of SRG to the L2 point, a thorough check-up of
subsystems of the Navigator platform and both telescopes as well as
calibrations of the telescopes were carried out. An extensive program of
observations of known point and diffuse X-ray sources with various spectral
and timing characteristics was carried out during the flight to L2 to
calibrate the alignment of the optical axes of the 14 mirror systems of the
telescopes (and their alignment with the axes of the satellite, of the optical
star trackers, and of the eROSITA and ART-XC telescopes) and to calibrate the
imaging and spectral characteristics of the detectors as well as parameters
relevant for timing studies of X-ray sources.

The main operation regime of the SRG orbital observatory is to scan the entire
sky in X-rays with the goal of constructing all-sky X-ray maps in several
energy bands. A detailed catalog of compact and diffuse X-ray sources in our
Galaxy and the extragalactic Universe is created thereby.

It was decided several years prior to launch that the first four years of the
SRG mission after the satellite’s arrival at the operational orbit near L2
will be devoted to an all-sky survey through eight consecutive scans of the
entire sky. This will allow the observatory to monitor the time variability of
numerous X-ray sources: its telescopes will for each sufficiently bright
source provide a light curve consisting of eight data points separated by half
a year. The total number of these bright sources may reach many tens of
thousands at energies between 300 eV and 2 keV and several hundreds at
energies above 4 keV. Among the most interesting X-ray transients expected to
be found by SRG in large numbers (for the first time) are tidal disruptions of
stars by supermassive and intermediate-mass black holes (Khabibullin et al.
2014; Malyali et al. 2019; Jonker et al. 2020).

Furthermore, the telescopes of the SRG observatory can trace the “fast”
variability of X-ray sources during the all-sky survey because each of the
eight data points in the long-term light curves will in fact consist of six
measurements separated by four hours. This capability should be useful, in
particular, for observations of afterglows of gamma-ray bursts (GRBs,
Khabibullin et al. 2012; Ghirlanda et al. 2015; Ascenzi et al. 2020), even if
a GRB is directed away from the Sun or obscured by Earth from the view of
near-Earth spacecraft (e.g., the Fermi observatory).

It is planned that after four years of continuous scanning of the sky and the
creation of eight independent X-ray maps of the entire sky, the SRG
observatory will switch for at least two years to a mode of detailed
observation of the most interesting X-ray sources in triaxial stabilization.
At this stage, long-term scanning of extended X-ray sources and selected
regions in the sky with an area of up to 150 square degrees will also be
possible. Table 1 summarizes the main parameters of the SRG observatory and
its telescopes. Further parameters of the ART-XC and eROSITA telescopes are
provided in Table 4 in §6 and Tables 5 and 6 in §7, respectively.

Table 1: Overview parameters of the SRG observatory and its telescopes.

Observatory
---
Orbit | Halo orbit around L2 point
| of the Sun–Earth system
Total mass | 2712 kg
Power consumption | 1700 W
Telemetry rate | 512 Kbit s-1
Pointing accuracy | $4\hbox{${}^{\prime\prime}$}$
Observation modes: | a) all-sky survey
| b) scanning (up to 150 sq. deg)
| c) triaxial pointing
Planned mission duration | 4 years all-sky survey and
|  $>2$ years pointed/scanning obs.
Telescope | eROSITA | ART-XC
Energy range | 0.2–8 keV | 4–30 keV
Energy resolution |  $\sim 80\,{\rm eV}$ at 1.5 keV | 9% at 13.9 keV
FoV (FWZI) | $\diameter=62\hbox{${}^{\prime}$}$ | $\diameter=36\hbox{${}^{\prime}$}$
Angular resolution |  $30$′′ HEW | 53′′ FWHM
(during survey) |  |
Pixel angular size | 9.6′′ | 45′′
Sensitivity | $<\mu{\rm Crab}$ | $\sim 100\mu{\rm Crab}$
(4-year all-sky survey) |  |

Notes: FWZI – full width at zero intensity, HEW – half-energy width, FWHM –
full width at half maximum.

The paper is organized as follows: in Section 2 we present the orbit and
operation regimes of the SRG observatory. In Section 3 we demonstrate the
sensitivities and capabilities of the two X-ray telescopes of the SRG and
discuss the heterogeneity of the X-ray maps obtained during the first two
scans of the entire sky. In Section 4 we describe the scientific goals and
first results of the mission. In Sections 5, 6, and 7 we describe the key
properties of the Navigator platform and of the ART-XC and eROSITA telescopes.
In Section 8 we briefly describe the history of the SRG project in Russia and
the responsibilities of the German and Russian consortia of scientists in the
processing and scientific analysis of the SRG/eROSITA data coming from two
complementary hemispheres of the sky. We finally draw our conclusions.

##  2 Orbit and operation regimes of the SRG observatory

Figure 5 shows the projection of the SRG orbit onto the ecliptic plane and the
scheme of the flight to the L2 point (Eismont et al. 2020). Orbital
corrections that were implemented before and after entry into the halo orbit
around L2 are also indicated.

Figure 5: Three projections of the SRG orbit onto the ecliptic planes and the
scheme of the flight to the L2 point. The dotted line shows the lunar orbit.
The points TCM1 and TCM2 denote major trajectory correction maneuvers of the
orbit (see §5.4). The point TCM3 denotes the insertion into the quasi-
stationary orbit around L2. The green and blue points indicate subsequent
minor corrections of the orbit. Courtesy of the Keldysh Institute of Applied
Mathematics of the Russian Academy of Sciences.

During the all-sky survey, the satellite rotates around an axis close to the
direction toward the Sun with a period of four hours. The rotation axis
gradually shifts by approximately one degree per day following the motion of
the Sun (see Fig. 6). As a result, the eROSITA telescope (with its 1 degree
field of view, FoV) observes each point source in the sky six times for 30–40
s over a day, but typically only once every six months. The full FoV of ART-XC
is 36 arcmin, so that a given celestial source is typically exposed four times
per day for $\sim 20$ s, also every six months. The variability of the sources
in the vicinity of the ecliptic poles could be monitored much longer. The SRG
observatory observes the entire sky and builds its full map every six months
(Fig. 7).

Star trackers are used for the precise orientation of the SRG observatory.
Both telescopes use star tracker data to refine their pointing accuracy. Fig.
8 shows the positions of the main star trackers that are installed on the two
telescopes of the observatory, as well as the position of the radio antennas
on the Navigator platform.

Figure 6: Rotation of SRG with a period of four hours around the axis pointed
at the Sun. This causes scans of large narrow circles in the sky. The survey
plane moves slowly, following the Sun at approximately 1 degree per day.

###  2.1 Inhomogeneity of the sky coverage during the survey

Figure 7: Beginning of the eROSITA all-sky survey. Initial test scans are
visible, and a dark stripe shows when there were no observations. Individual
scans cross at the north and south ecliptic poles. The inset shows a
$2^{\circ}\times 2^{\circ}$ region within the deep field near the north
ecliptic pole, containing the cluster of galaxies A2255 and a bright quasar.

The adopted strategy of the SRG survey leads to the appearance of deep fields
around the north and south ecliptic poles in the sky map (see Fig. 7), where
the large circles of all individual scans cross. As a result, the exposure
time depends on the ecliptic latitude $\theta$ as $1/\cos\theta$, that is, the
exposure is shortest at the ecliptic equator and longest at the ecliptic
poles.

Figure 8: SRG spacecraft. Positions of the key star trackers, two
omnidirectional radio antennas and a medium-gain antenna for transmitting
scientific data. Figure 9: Rotation speed of the Z-axis of the SRG spacecraft
in the ecliptic plane during the first two sky surveys. The spacecraft spins
around the Z-axis with a period of four hours, thus performing the all-sky
survey. Due to limitations on orientation of the spacecraft, the angle between
the Z-axis direction and the Sun and Earth must not exceed 13 and 24 degrees,
respectively. This condition defines the speed with which the Z-axis has to
follow the Sun and Earth at any point in the halo orbit around L2. Variation
in the speed of the Z-axis causes variations in the exposure of the survey.
Higher speed leads to lower exposures, and lower speed allows higher exposures
of the corresponding sky regions. The vertical bars mark the dates of the gaps
in the all-sky survey that are associated with orbit corrections and
calibrations. Figure 10: Exposure map (in Galactic coordinates) of the first
all-sky survey by the ART-XC telescope. Exposure time is given in seconds (see
the color scale shown on the right). The white spots in the map correspond to
the ecliptic poles. Exposure times for eROSITA are approximately three times
longer than for ART-XC, in accordance with the instruments’ FoV (36′ and
1.03∘, respectively). Figure 11: Distribution of eROSITA exposure (upper
panel) and sensitivity in the 0.3–2.2 keV energy band (lower panel) after the
first three all-sky surveys. The solid curve in the upper panel shows the area
of the sky in which the eROSITA exposure exceeded the given value. The solid
curve in the lower panel shows the area of the sky in the hemisphere analyzed
by the Russian consortium in which the $3\sigma$ sensitivity is better than
the given value.

During the first two all-sky surveys, additional inhomogeneity in the sky
exposure results from the SRG elongated (in the ecliptic plane) halo orbit
around L2. The distance to the Sun varies significantly over six months (see
Fig. 5). The satellite moves ahead of Earth for three months and lags Earth
for the next three months, which causes a significant variation in drifting
rate of the scanning plane, from 0.7 deg/day at the closest to the Earth
segment of the orbit to 1.6 deg/day at its farthest segment (see Fig. 9).
Figure 10 shows the actual exposure map for the first SRG all-sky survey based
on ART-XC data.

Figure 11 (upper panel) shows the sky area covered by eROSITA with a given
exposure. After the first three all-sky surveys, half of the sky has been
covered with an exposure of at least 600 s, while a total area of $\sim 3$
square degrees around the ecliptic poles has been covered with an exposure
exceeding 50 ks, so that source confusion is already high in these regions.
The lower panel of Fig. 11 shows the corresponding sensitivity achieved by
eROSITA in the 0.3–2.2 keV energy band after the first three all-sky surveys.

###  2.2 Deep scanning of sky fields with a size up to 150 square degrees

Figure 12: Sky visualization of a typical route of the X-axis in scanning
mode.

During the final stages of the flight to the L2 point, long observations of a
large number of point sources and extended Galactic and extragalactic sources
were carried out for calibration (Cal) and performance verification (PV) of
the SRGtelescopes. In addition, scanning deep-survey observations were
performed of a number of extended extragalactic fields, a region of the
Galactic X-ray Ridge, and fields in the direction of nearby molecular clouds
(to reveal X-ray diffuse emission between them and us). The targets for the PV
phase have been chosen by the ART-XC science team and the science working
groups of the German and Russian eROSITA consortia (see examples of the maps
obtained during these observations in §4.1,4.2 below).

To perform deep surveys of extended (up to 150 square degrees) fields, NPOL
and IKI proposed and implemented a novel fruitful method of scanning
observations, described in §5 below and illustrated in Fig. 12. This method
provides a much more uniform coverage of the observed field compared to the
standard method, which is based on a grid of pointed observations, and enables
obtaining high-quality maps of extended astrophysical objects.

###  2.3 Pointed observations

The Navigator platform also allows observations of chosen targets to be done
with the triaxial orientation of the observatory. This regime was successfully
tested during flight calibration and PV observations. The pointing precision
is described in §5 below.

###  2.4 Instrumental background at L2

The particle background recorded by both instruments at L2 has been notably
stable during the first 1.5 years of operations, when the solar activity was
very modest. This is illustrated in Fig. 13, where the eROSITA count rate in
the 7–9 keV band is shown. In this energy range, the internal detector
background dominates the astrophysical background. The horizontal dashed red
line shows the typical quiescent background. There are rare spikes of counts
with a modest amplitude, which are most likely caused by low-energy charged
particles reaching the CCD through the thin entrance filter.

Figure 13: eROSITA count rate (per single detector) in the 7–9 keV energy band
during the first ten months of the all-sky survey in 6000 s bins. In this
energy band, the detector background dominates astrophysical sources. The
horizontal dashed red line shows the typical quiescent background, which is
globally very stable. Short spikes, whose amplitude rarely exceeds 10-20%, are
plausibly associated with low-energy charged particles, which reach the CCDs
through the thin entrance filters. Figure 14: Sketch of ART-XC (dashed lines)
and eROSITA (solid lines) astrophysical and internal detector backgrounds. The
blue lines show the estimated sky background using the models of Lumb et al.
(2002) and Gruber et al. (1999), convolved with the telescopes’ responses
averaged over the FoV. The red lines show the level of the detector internal
background (fluorescent lines are not shown). For eROSITA, the astrophysical
background dominates below $\sim 2$ keV, while at higher energies, the
detector background exceeds the sky background by an order of magnitude.

The spectrum of the eROSITA detector background is described in Predehl et al.
(2021). Here, we show a simplified version of the background model for both
instruments (see Fig. 14). In this figure, the blue lines show the (slightly
modified) model of sky astrophysical background (Gruber et al. 1999; Lumb et
al. 2002) convolved with the telescopes’ responses. For comparison, the red
lines illustrate the level of the internal background of the detectors
(excluding fluorescent lines). For eROSITA, the astrophysical background
dominates below $\sim 2$ keV the astrophysical background, while at higher
energies, the detector background is the factor that affects the sensitivity
of the telescope. Figure 14 also demonstrates that the particle background in
both instruments is comparable.

Figure 15: eROSITA image of the region around the extremely bright Galactic
black hole binary Cygnus X-1 in the 0.4–4 keV energy band. The image was
saturated in the core to make very faint diffuse structures visible. In
particular, the extended halo that is visible up to $\sim 3$ degrees from the
source is due to X-ray photons scattered by mirrors only once (stray light).
The same effect is largely responsible for a halo around Sco X-1 that is
visible in the all-sky map. A special baffle mounted on top of eROSITA mirrors
strongly reduces the magnitude of stray light, so that it is visible only
around extremely bright objects. The radial rays that are clearly seen in the
image are due to the mirror support structure.

###  2.5 Role of stray light

The eROSITA telescope optics (Wolter-I scheme) focuses photons that are
scattered two times: first by parabolic shells, and then by hyperbolic shells.
However, some photons can reach the detector after only one scattering or
after two scatterings involving “wrong scatterings” , for instance, when they
are scattered by the outer surface of a shell. All these photons are
collectively called “stray light”. A special baffle was introduced to reduce
the magnitude of the stray light by an order of magnitude (Friedrich et al.
2014; Predehl et al. 2021). Nevertheless, a faint and extended halo associated
with stray light can be seen when extremely bright sources are observed, such
as the X-ray binary Cygnus X-1 (Fig. 15). The role of stray light is more
significant in the case of ART-XC (see §6.2 below and Pavlinsky et al. 2021b
for details).

##  3 Sensitivity of the telescopes of the SRG observatory

Figure 16 compares the on-axis effective areas of the eROSITA and ART-XC
telescopes as a function of the energy of registered photons. The eROSITA and
ART-XC curves intersect near 5 keV. The telescopes nicely complement each
other, detecting X-ray photons in adjacent energy bands. The results of the
first all-sky survey have confirmed the preflight estimates of the effective
area of the SRG telescopes.

Figure 16: On-axis effective area of the eROSITA (red) and ART-XC (blue)
telescopes of the SRG observatory. These curves are based on the pre-flight
beam tests and models, which are broadly consistent with the in-flight
calibration results.

Figure 17 shows the sensitivity of the eROSITA and ART-XC telescopes that was
achieved during the first half-year survey as a function of energy. The
sensitivity has been evaluated assuming an unabsorbed power-law spectrum with
a photon index of 1.8 in the energy interval $\Delta E\sim E$. The in-flight
background spectrum (both the intrinsic detector background and unresolved
astrophysical sky emission, see Freyberg et al. 2020 for details) is
integrated over this energy interval to predict the image surface brightness.
Using this value, we can predict the expected distribution of fluxes (due to
Poisson fluctuations of the number of counts) at a given position of an image
convolved with the telescope point spread function (PSF). The sensitivity is
then estimated as the flux associated with peaks that for a Gaussian
distribution would correspond to 5$\,\sigma$ deviations. The corresponding
curves are labeled S1. In the survey, the exposure is shortest near the
ecliptic equator, where it is 200 and 60 s for eROSITA and ART-XC,
respectively. The difference in exposure is due to the difference in the solid
angles subtended by the two telescopes at any given moment. The horizontal
bars show the broadband sensitivities in the first survey, approximately
converted into units of flux density.

Figure 17: Estimated sensitivities of the ART-XC (red) and eROSITA (blue)
telescopes for a continuum spectrum in the energy interval $\Delta E\sim E$ as
a function of energy. The curves show the sensitivity in units of energy flux
density, i.e., ${\rm erg\,s^{-1}\,cm^{-2}\,keV^{-1}}$, evaluated for an
interval of energies between $E_{1}=E/f$ and $E_{2}=E\times f$, where $f=1.6$.
The top curves, labeled S1, correspond to the sensitivity achieved during the
first all-sky survey close to the ecliptic equator, where the exposure time is
shortest. The difference in the effective time (200 s vs 60 s for eROSITA and
ART-XC, respectively) reflects the difference in the FoVs of the two
instruments. Also shown are the sensitivity curves for 10 and 100 times deeper
exposures. The horizontal bars show the broadband (0.2–2.3 keV and 2.3–8 keV
for eROSITA and 4–12 keV for ART-XC) sensitivities in the first survey,
approximately converted into units of flux density. The spectrum of the Crab
nebula scaled to a flux of 1 mCrab and 1 $\mu$Crab is shown for comparison.

##  4 Science goals and first results of the SRG observatory

The main scientific objective of the SRG observatory is the construction of
detailed X-ray maps of the sky and catalogs of point sources and extended
X-ray sources in different energy bands from 0.3 to 12 keV. The ART-XC and
eROSITA telescopes were developed specifically for performing these tasks.

It is expected that during the eight all-sky surveys spanning a period of four
years, up to four to five million compact X-ray sources will be discovered:
about three million active galactic nuclei (AGN), nearly one hundred thousand
rich clusters of galaxies, up to a million stars with bright coronae (mostly M
dwarfs), and tens of thousands of other Galactic objects, including cooling
neutron stars and radiopulsars, accreting neutron stars (in particular, X-ray
pulsars and bursters), black holes, and numerous white dwarfs in binary
stellar systems.

Also of interest is the exploration of extended X-ray sources, including
supernova remnants (SNRs), pulsar wind nebulae, the hot gas filling most of
the interstellar volume near the Galactic plane, rarefied gas in the halo of
the Galaxy, and gas fountains in nearby galaxies. In addition, there is
interest in the Local Bubble, comets in the Solar System, and in a search for
traces of a shock wave at the boundary of the heliosphere, where the solar
wind is halted by the interstellar gas surrounding the Solar System. It is
also possible to continue the investigation that was begun by ROSAT (Snowden
et al. 1997) of the origin of the Galactic soft X-ray background, which
depends on the distribution of molecular and atomic gas and dust in the
Galaxy, which absorb the soft X-ray emission.

As demonstrated by the highly successful ROSAT spacecraft (all-sky X-ray
survey in 1990, Voges et al. 1999), these data are eagerly anticipated and
will be widely used by the global astronomical community. Furthermore, the
availability of eight all-sky surveys, each lasting six months, should help
the SRG observatory to discover a huge number of variable compact sources of
Galactic and extragalactic origin.

###  4.1 X-ray maps obtained during the first all-sky survey and the CalPV
phase

Figure 18: Positions (in Galactic coordinates) of the X-ray sources detected
by SRG/ART-XC during its first all-sky survey (typical exposure per point is
just $\sim 60$ s). Nearly 600 sources have been detected in the 4–12 keV
energy band, $\sim 60$% of which are Galactic (black holes, neutron stars,
white dwarfs, coronally active stars, SNRs, etc.) and $\sim 40$% are
extragalactic (AGN and a few dozen massive clusters of galaxies), as well as a
number of newly discovered sources. The symbol size reflects X-ray brightness
of the sources. Figure 19: Positions (in Galactic coordinates) of X-ray
sources detected by SRG/eROSITA in the 4–8 keV energy band during the first
two all-sky surveys on that half of the sky in which the Russian Consortium is
responsible for the data analysis (the Galactic center is on the right side of
the image). The image includes $\sim$600 sources, and some of the brightest
are labeled.

Figure 18 shows the distribution over the sky of the $\sim 600$ sources
detected by ART-XC in the 4–12 keV energy band during its first sky survey
(December 12, 2019 – June 10, 2020). Recently, an updated version of this map
has been presented by Pavlinsky et al. (2021a), which is based on the sum of
the first two sky surveys (December 2019 – Dec. 2020). A total of 867 sources
(821 point sources and 46 extended sources) have been detected by ART-XC. The
750 sources of known or suspected origin in this catalog include 56%
extragalactic sources (mostly AGN and also 52 rich low-redshift clusters of
galaxies), and the rest are Galactic (X-ray binaries, cataclysmic variables
(CVs), SNRs, etc.). From 114 sources, ART-XC has detected X-rays for the first
time. Although the majority of them ($\sim 80$) are expected to be spurious
given the chosen detection threshold, there are expected to be $\sim 35$ newly
discovered astrophysical objects. An ongoing program of optical follow-up
observations of these sources has already led to the identification of several
new AGN and CVs (Zaznobin et al. 2021b, a).

The achieved sensitivity to point sources after the first year of the ART-XC
all-sky survey varies between $\sim 4\times 10^{-12}$ erg s-1 cm-2 near the
ecliptic plane and $\sim 8\times 10^{-13}$ erg s-1 cm-2 (4–12 keV) near the
ecliptic poles. The typical depth of the ART-XC survey is already comparable
to that achieved in a similar energy band (4–10 keV) in the recent MAXI
(Monitor of All-sky X-ray Image) all-sky survey (Kawamuro et al. 2018) and is
just slightly poorer than the sensitivity of the XMM-Newton Slew Survey in the
2–12 keV band (Saxton et al. 2008). However, the ART-XC survey greatly
improves on the former in terms of angular resolution and provides full and
regular sky coverage in contrast to the latter. The ART-XC survey sensitivity
will be growing during the mission as a result of increasing exposure and
probably also due to a decreasing flux of Galactic cosmic rays as the next
solar maximum approaches. As many as $\sim 5000$ sources can be found by ART-
XC by the end of the four-year all-sky survey.

As noted above, the eROSITA telescope is noticeably inferior in sensitivity to
the ART-XC telescope at energies above 5–6 keV, but between 4 and 5 keV,
eROSITA is more sensitive. As shown in Fig. 19, during the first two surveys
of the sky, eROSITA detected over 600 X-ray sources in one hemisphere in the
range from 4 to 8 keV. This number is comparable to the number of sources
detected by ART-XC over the entire sky during one survey in the 4–12 keV
range.

Figure 20: Annotated version of the SRG/eROSITA first all-sky image. Several
prominent X-ray features are marked, ranging from distant galaxy clusters
(Coma, Virgo, Fornax, and Perseus) to extended sources such as SNRs and
nebulae to bright point sources, e.g., Sco X-1 (the first known extrasolar
X-ray source). The map of the whole sky is constructed by the two scientific
consortia of SRG/eROSITA in Germany and Russia. Each consortium created their
images for one half of the sky. The map is an RGB map, where photons of
different energies are shown in different colors: from 300 to 600 eV in red,
from 600 eV to 1.0 keV in green, and from 1 to 2.3 keV in blue. The colors on
this map, obtained from about 400 million photons registered by eROSITA during
six months of the survey, allow one to immediately judge the temperature of
radiating gas, ranging from 3 to more than 10 million K. At the very center of
the map, the supermassive black hole Sgr A* is located with a mass of about 4
million solar masses. It is a rather weak X-ray source and is nearly invisible
on this map. In the middle plane of the picture lies the disk of the Milky
Way. It looks dark because the molecular gas and dust in the plane of the
Galaxy absorb X-rays. The blue dots located in this region reveal a large
number of bright and powerful X-ray sources in the Milky Way: X-ray pulsars,
accreting black holes, neutron stars, white dwarfs in binary stellar systems,
and remnants of supernova explosions. Figure 21: Optical spectrum of quasar
SRGE J170245.2+130107 (discovered by SRG/eROSITA) obtained with the BTA 6m
telescope. The light gray and dark gray lines show the spectra obtained on
August 17, 2020, and September 13, 2020, respectively, while the combined
spectrum is shown by the black line. The red dots show the source flux density
in the Pan-STARRS $r,i,z,y$ filters. The vertical dashed lines show the
expected positions of the peaks of the emission lines of the quasar at
$z=5.466$. Adapted from Khorunzhev et al. (2021).

The eROSITA telescope has obtained the best map of the sky in the history of
X-ray astronomy in the 0.3–2.3 keV energy band (Fig. 20) even after its first
all-sky survey. The good angular resolution and high sensitivity of eROSITA
allowed it to detect over a million compact sources and map about 20 thousand
extended sources. This huge number of sources cannot be displayed in a single
image; only the brightest of them are visible on the map as dots. After
scanning the sky for just six months, eROSITA has constructed a map that is
about four times more sensitive and contains almost eight times more sources
than the previously best all-sky map that was obtained in 1990 by the ROSAT
satellite. eROSITA has already nearly doubled the total number of sources
detected by all orbital observatories over the $\sim 60$ years of X-ray
astronomy.

The eROSITA map reveals spectacular giant bubbles of hot gas with temperatures
up to 10 million K expelled from the plane of the Galaxy. These are the
outcome of hundreds of thousands of supernova explosions and/or the
intermittent activity of the supermassive black hole in the center of our
Galaxy (Predehl et al. 2020). These X-ray bubbles are seen above and below the
midplane of the image that encloses the well-known Fermi bubbles, which are
associated with gamma-rays emitted through the interaction of cosmic rays with
the ambient gas (Ackermann et al. 2014).

About three-quarters of the objects in the SRG/eROSITA map are AGN that are
powered by the accretion of matter onto supermassive black holes residing in
their centers. They are located far beyond the Milky Way. The quasar CFHQS
J142952+544717 at $z=6.2$ (corresponding to the age of the Universe of 900
million years) is particularly interesting. It was detected in X-rays for the
first time by eROSITA and proved to have the highest X-ray luminosity ($\sim
3\times 10^{46}$ erg s-1, or $\sim 10^{13}$ bolometric luminosities of the
Sun) of the quasars at $z>6$ (Medvedev et al. 2020, 2021). During the
calibration and performance verification (CalPV) phase and in the course of
scanning the entire sky, SRG/eROSITA has discovered a number of extremely
luminous quasars at redshift $z>5$ and a noticeable number at $z>4$
(Khorunzhev et al. 2020; Dodin et al. 2020; Bikmaev et al. 2020; Wolf et al.
2021). In Fig. 21 we show the spectrum of such a quasar discovered by
SRG/eROSITA at $z=5.5$. It was obtained with the 6-meter BTA telescope in the
North Caucasus (Khorunzhev et al. 2021). The majority of the $\sim 20000$
extended objects in the eROSITA map are clusters of galaxies filled with dark
matter and hot intergalactic gas, which shines in X-rays. Fewer than 50% of
these clusters were previously known from optical surveys or due to detection
of the Sunyaev–Zeldovich effect (Sunyaev & Zeldovich 1980) in their direction
by the Planck spacecraft (Planck Collaboration et al. 2016), the Atacama
Cosmology Telescope (ACT, Hilton et al. 2021), or the South Pole Telescope
(SPT, Bleem et al. 2015) sky surveys.

There will be much synergy and competition between different methods of
observation of galaxy clusters over the next years through the continuation of
sky surveys in different spectral bands, including the SRG survey in X-rays
and ground-based surveys by ACT and SPT in microwaves. Not only will the
results of these surveys be valuable for cosmological studies, but we should
also expect the discovery of thousands of strong gravitational lenses due to
the deep gravitational potential of clusters of galaxies.

Some two hundred thousand fairly close stars with coronae much more powerful
than the corona of the Sun also contribute to the emission from the zones of
relatively low temperature in the eROSITA sky map (Fig. 20). Interestingly,
eROSITA has detected X-ray emission from 150 stars with known exoplanets. This
amounts to some 10% of all nearby stars with known planetary systems
(excluding the more distant stars with exoplanets in the field that were
explored by the Kepler satellite).

Figure 22: SRG/eROSITA maps of one half of the sky in the 0.3–0.7 and 0.7–2.3
keV energy bands.

Figure 22 demonstrates the dramatic difference between images of the sky (one
hemisphere) in the 0.3–0.7 keV and 0.7–2.3 keV energy bands. In the 0.3–0.7
keV band, the emission is fairly homogeneous, bright, and diffuse. This might
be the background radiation of a huge number of soft X-ray sources located at
cosmological distances and/or emission from the hot ($10^{5}<T_{\rm e}<10^{7}$
K) gas in the halo of our Galaxy. An additional significant contribution to
the sky brightness at these energies might be provided by relatively nearby
sources such the hypothetical Local Bubble that was created by supernova
explosions in the relative vicinity of the Solar System. The strong absorption
of soft X-rays by the cold atomic and molecular gas and dust in the Galactic
plane is also noteworthy. A very different picture is observed in the 0.7–2.3
keV energy band. Here the image is dominated by many hundreds of thousands of
extragalactic sources. The main contribution to the source counts and to the
diffuse background is provided by AGN. The absorption by cold gas and dust in
the Galactic plane is strongly reduced by the rapidly decreasing
photoabsorption cross-section with increasing photon energy. Bright Galactic
sources in the region of active star formation in the Cygnus constellation are
clearly visible in both maps.

Figure 23: Fragment ($\sim 3^{\circ}\times 2^{\circ}$) of an image of the
central region of the Galaxy obtained by ART-XC in the 4–12 keV energy band
during the CalPV phase. Figure 24: Image ($\sim 40$ arcmin on a side) of the
SNR RX J1713.7$-$3946 obtained by ART-XC in the 4–12 keV energy band during
the CalPV phase.

###  4.2 Examples of the results of deep surveys during the PV phase

####  4.2.1 Galactic center

During the CalPV phase, ART-XC performed deep surveys of a number of extended
fields in the sky, in particular, of a large ($\sim 40$ square degrees) region
in the center of our Galaxy. Figure 23 shows a fragment of the image obtained
in the 4–12 keV energy band, which reveals the high quality and richness of
the data.

All in-flight characteristics of ART-XC have proved to be close to preflight
expectations (see more details in §6.4). In particular, the PSF averaged over
the FoV is better than 1 arcmin (half-power diameter, HPD) in survey mode. The
good angular resolution of the telescope is evident from the image (Fig. 24)
of the SNR RX J1713.7$-$3946 that is clearly resolved by ART-XC.

####  4.2.2 Galactic Ridge

Figure 25 demonstrates a large variety of astrophysical objects of our Galaxy
that are accessible for observation by the SRG observatory. During scans of
the Galactic Ridge (in a field $\sim 20$ degrees away from the Galactic
center), eROSITA has detected stars with active X-ray emitting coronae, star-
forming regions and clusters of young stars, X-ray pulsars (rapidly rotating
magnetized neutron stars), and SNRs. In the latter, X-ray photons are emitted
by gas that is compressed in shocks where the material of the exploded star
collides with the surrounding interstellar matter. As is well known, hot gas
occupies 80–90% of the volume near the Galactic plane, and just 10–20% of the
volume is filled by dense clouds of cold molecular and atomic hydrogen.

Figure 25: Annotated X-ray RGB map of a 25 square degrees region in the disk
of the Milky Way (the so-called Galactic Ridge) obtained by eROSITA in October
2019. In this field, thousands of Galactic X-ray sources are detected, as well
as a number of quasars observed through the Galactic disk. In addition to many
individual sources, the map also shows unresolved X-ray emission from hot gas
and from a multitude of faint unresolved sources. Blue and green correspond to
high photon energies emitted by a gas with a temperature of tens of millions
of degrees, while red regions reveal colder gas of lower temperature. Adopted
from Gilfanov, Medvedev, Sunyaev et al. 2022 (in preparation). Absorption in
the cold interstellar gas near the Galactic plane attenuates soft X-ray
radiation, but allows hard X-ray photons to leak through because their
absorption cross-section is smaller. A significant contribution to the
apparently diffuse emission from the Galactic disk and bulge is provided by
the superposition of emission from numerous accreting white dwarfs, hot
coronae of low-mass stars, and flares on them (Revnivtsev et al. 2009).
eROSITA cannot resolve individual contributions of these sources because of
their great number and low luminosities.

####  4.2.3 Lockman Hole

Figure 26: X-ray image of the Lockman Hole obtained by eROSITA: More than
8,500 X-ray sources in 18 square degrees. Adopted from Gilfanov, Burenin,
Sunyaev et al (in preparation).

Figure 26 demonstrates the richness of the extragalactic X-ray sky revealed
during a long scan (about 8 ks per pixel) of the Lockman Hole zone. In this
unique region, the absorption of X-rays by the interstellar medium of the
Galaxy is close to its minimum in the entire sky. This allows studies of
distant quasars and clusters of galaxies in unprecedented detail. In the
$\approx 20$ square degrees field, eROSITA has detected over 8,500 point X-ray
sources, that is, $\sim 400$ sources per square degree. This number
corresponds to $\sim 16$ million objects when extrapolated to the whole sky.
The vast majority of these sources are AGN. According to photometric redshift
estimates, the most distant of the quasars detected by eROSITA in the Lockman
Hole are located at $z\sim 5$. Some $\sim 200$ clusters and rich groups of
galaxies filled with hot gas were detected. In addition, several hundred
active Galactic stars are also seen in the Lockman Hole field.

####  4.2.4 Coma cluster

Figure 27:  X-ray image of the Coma cluster (Churazov et al. 2021) in the
0.4–2 keV band obtained by eROSITA in the course of the CalPV program (top
left). The image is $\sim$6 degrees on a side, corresponding to 10 Mpc at the
distance of the cluster, with the logarithmic color-code spanning five orders
of magnitude. The main cluster is in the process of merging with the NGC 4839
group (a bright blob in the bottom right corner from the Coma cluster).
Flattened X-ray image of the Coma cluster field (bottom left). The labels
schematically mark some of the features that are presumably associated with
the merger with the NGC 4839 group. The dashed blue line is the suggested
trajectory of the group, which enters the Coma cluster from the northeast
direction, and is currently close to apocenter. The presumed positions of two
shocks driven by the NGC 4839 group are shown with the red and purple curves.
The shock closer to the center is driven by the displaced gas that settles
back to hydrostatic equilibrium. This is the most salient feature that can be
directly seen in the image at the surface brightness edge. The green line
shows the faint X-ray bridge connecting NGC 4839 and the main cluster, which
is a possible trace of the group passage through the Coma cluster. The yellow
line shows the contact discontinuity, which is an interface between cold and
hot gas patches with the same pressure. See Churazov et al. (2021) for
details. ART-XC image of the Coma Cluster in the 4–12 keV energy band (top
right). Planck spacecraft $y$-parameter map of the Coma cluster, based on the
data in the microwave spectral band (bottom right). The morphology of this
map, which reflects the distribution of the hot gas electron pressure, is
strikingly similar to the eROSITA image in the X-ray band.  Figure 28:
Preliminary $\log(N)-\log(S)$ distributions of compact sources in the
extragalactic part of the hemisphere, where the Russian consortium of
scientists is responsible for the data analysis and interpretation. The
distribution of sources with and without Gaia stars among their optical
counterparts are shown by blue and red histograms and are marked in the plot
legend as GAIA stars and AGN, respectively. The distributions are derived from
partial data of the first two sky surveys and do not use the full potential of
the first-year data. No incompleteness correction was applied at the faint
flux end. The coefficient $A$ on the y-axis incorporates approximations and
inaccuracies of the preliminary analysis. It is reasonably close to unity and
may weakly depend on flux, which is not necessarily the same for GAIA stars
and AGN.

One of the main targets of the CalPV phase was the Coma cluster (Fig. 27). SRG
observations in the scanning mode are particularly well suited for a detailed
mapping of this massive and nearby cluster well beyond its virial radius. The
obtained ART-XC data enable measuring the temperature of the hot gas and
estimating the contribution of cosmic rays to the observed X-ray emission,
while eROSITA images provide detailed information about the merger of the Coma
cluster with its less massive companion, the galaxy group NGC4839. Fig. 27
clearly shows the rich substructure that is generated by the merger, including
shock waves and contact discontinuities extending over a few megaparsec. The
data from the Planck observatory (distribution of brightness in the microwave
band due to the SZ effect) provide additional information about the
distribution of pressure in the space between galaxies (Planck Collaboration
et al. 2013). The comparison of the data from the three telescopes provides a
detailed picture of the distribution of hot gas and dark matter in the cluster
and its outskirts.

It is important to note that in the Coma cluster field, eROSITA has unveiled
many dozen massive clusters of galaxies and thousands of quasars located at
cosmological distances far beyond the Coma cluster. This is a consequence of
the deep scanning of a field with a size of $3^{\circ}\times 3^{\circ}$.

###  4.3 Source catalog

The source catalogs are maintained by the Russian (RU) and German (DE)
consortia for the respective sky hemispheres. A preliminary version of the
$\log(N)-\log(S)$ distribution of compact sources in the extragalactic part of
the RU sky is shown in Fig. 28

###  4.4 X-ray spectroscopy

During the CalPV phase, ART-XC performed long pointed observations of a number
of bright X-ray sources, including the well-known nearby AGN Circinus galaxy.
The obtained spectrum (Fig. 29) shows a complex continuum in the 4–20 keV
energy band along with a strong Fe-K$\alpha$ emission line. The estimated
energy resolution of $\sim 1.3$ keV at 6 keV matches the preflight estimates.

Figure 29: X-ray spectrum of the Circinus galaxy measured by ART-XC. The
Fe-K$\alpha$ line is prominent on top of the complex continuum. The exposure
is about 50 ks. Flux units are arbitrary.

Figure 30: 0.4–2 keV X-ray image of the SNR Cygnus Loop obtained by eROSITA
during the first two SRG all-sky surveys (upper panel). The longest exposure
time (per point) across the image is $\sim$400 s. The axes are labeled in
degrees. X-ray spectrum of the entire Cygnus Loop (lower panel). The ions
contributing to some of the brightest emission lines are indicated. The
spectrum is normalized per square arcminute. Figure 31: Spectrum of the
Galactic microquasar SS 433 obtained during the SRG/eROSITA PV phase. The data
are well fit by the model of baryonic multitemperature jets (Khabibullin et
al. 2016), with contributions of the approaching (blueshifted) and receding
(redshifted) jets shown in blue and red, respectively. The positions of the
the lines of hydrogen- and helium-like silicon, sulphur, iron, and nickel are
marked.

The excellent capabilities of the eROSITA telescope in spectroscopy of hot
astrophysical plasmas (temperatures of some million degrees) in bright X-ray
sources during the all-sky survey are clearly demonstrated (Fig. 30) by the
spectrum of the central zone of the Cygnus Loop SNR obtained using an exposure
of just 200 s. X-ray emission lines of a number of ions of various chemical
elements are clearly seen. The spectroscopic capabilities of eROSITA in the
3–9 keV band are demonstrated in Fig. 31, which shows the spectrum of the
Galactic microquasar SS 433 obtained during the SRG/eROSITA PV phase. Emission
lines of highly ionized atoms (silicon, sulphur, iron, and nickel)
corresponding to the approaching (blueshifted) and receding (redshifted)
baryonic multitemperature jets are clearly resolved. The line positions are
fully consistent with the expectations based on the kinematic precession model
with the bulk velocity of the jets equal to a quarter of the speed of light.

###  4.5 X-ray timing

Figure 32: Crab phase histograms reconstructed in several energy bands. The
ART-XC exposure is about 45 ks. Figure 33: eROSITA spectrum of tidal
disruption event SRGe J213527.3$-$181634/ZTF20abgbdpr. The best-fit multicolor
accretion disk emission model with the inner disk temperature of $\sim 100$ eV
is shown by the dashed line (Gilfanov et al. 2020).

The ART-XC telescope has an excellent time resolution of 23 $\mu s$, which is
determined by the resolution of time stamps assigned to incoming events by the
reading electronics. This permits a detailed study of the time behavior of
millisecond pulsars in pointing mode. The timing capabilities of the telescope
were tested during observations of the Crab pulsar. Figure 32 shows the light
curve of this pulsar in several energy bands as measured by ART-XC and folded
with the Jodrell Bank radio ephemeris. The $\sim 33$ ms pulsations are clearly
detected up to 30 keV, and the energy dependence of the pulse morphology is as
expected.

The nominal integration time of the eROSITA CCDs is 50 ms. This integration
time is set by the requirement that smearing of X-ray images in the all-sky
survey is to be avoided. With the nominal rotation rate (360 degrees in four
hours), the sources in the telescope FoV shift by $3.5"$ in 50 ms, which is
about three times smaller than the CCD pixel size. While this integration time
precludes a systematic study of the variability on millisecond scales, the
stability of the background and instrument characteristics provides an
excellent opportunity for monitoring objects on longer timescales in three
important intervals: 0.05–40 s (for sources crossing the FoV during a single
scan), four hours to one day (during six intra-day scans of the same object),
and on scales from six months to four years (over the entire SRG all-sky
survey). In addition, the variability of sources located close to the
intersections of individual scans near the ecliptic poles can be tracked
nearly continuously on scales longer than four hours.

###  4.6 ART-XC Galactic transients

In its daily scans (exposure of $\sim 60$ s per source), the ART-XC telescope
reaches a sensitivity ($\sim 5\sigma$) of $\sim 8\times 10^{-12}$ erg cm-2 s-1
in the 4–12 keV energy band (about 0.6 mCrab). By surveying about $1\%$ of the
sky every day, ART-XC provides rapid alerts about new X-ray transients (e.g.,
Mereminskiy et al. 2020c, b; Schwope et al. 2020) or new outbursts from known
or poorly studied historical sources (e.g., Mereminskiy et al. 2020d, a).
These events are typically also detected by eROSITA, which provides a more
accurate localization for newly discovered sources and detailed spectral
information below $\sim 2$–8 keV for a broadband spectral analysis.

The ART-XC telescope provides the unique possibility of studying the
population of faint transients that would be otherwise missed because they are
too weak for wide FoV telescopes and all-sky monitors [such as the
INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL)/IBIS, Swift/BAT,
and MAXI]. The relatively hard X-ray band of the ART-XC telescope also makes
it less dependent on the source intrinsic or Galactic absorption and allows
the detection of highly absorbed sources, which could be missed by soft X-ray
instruments. For example, ART-XC transients often appear quite unremarkable in
eROSITA data. Follow-up campaigns have already enabled establishing the nature
of several ART-XC sources, such as the new microquasar SRGA
J043520.9+552226/AT2019wey (Yao et al. 2020b, a; Mereminskiy et al. 2021), the
new nova-like CV SRGt J062340.2$-$265715 (Schwope et al. 2021), several new Be
systems (Doroshenko et al. 2021; Lutovinov et al. 2021), and a few others.

###  4.7 Extragalactic transients and stellar flares

The approach of repeated sky surveys when every (typical) location on the sky
is visited every six months has proven to be an efficient tool for studying
the long-term variability of sources and for discovering various types of
Galactic and extragalactic transients. Every 24 hours, eROSITA detects between
half a dozen and a dozen sources that have changed their luminosity by more
than an order of magnitude compared to the previous visit six months earlier
(e.g., Gilfanov et al. 2020; Sazonov et al. 2020; Medvedev et al. in
preparation). About half of these sources are associated with Gaia stars, and
the remaining half are presumably of extragalactic origin. The requirement of
at least a tenfold flux increase corresponds to an effective transient
detection threshold of $\sim 2\times 10^{-13}$ erg/s/cm2.

As every scan path crosses the ecliptic poles, sources in these regions are
repeatedly scanned every four hours. For these sources, we have already
accumulated light curves covering over 16 months, which opens unique prospects
of detailed variability studies of stars and AGN on timescales from several
hours to a few years (Medvedev et al. in preparation). The sky scans are
planned to proceed in an unchanged manner at least until the end of the third
sky survey.

Tidal disruption events (TDEs) are being actively searched for among
extragalactic eROSITA transients based on the optical and infrared properties
of their hosts and on the shape of their X-ray spectra (Fig. 33). The Russian
consortium identifies about one relatively bright TDE candidate every week
(Khabibullin et al. 2020b, a; Gilfanov et al. 2020, 2021). They are followed
up with optical spectroscopy on various telescopes in Russia (the 6 m BTA
telescope in the Caucasus; the 1.5m Russian-Turkish telescope, RTT-150, in
Turkey; the 1.6m AZT-33IK telescope of the Sayan Observatory; and the 2.5m
telescope of the Caucasus Mountain Observatory of the Sternberg Astronomical
Institute of the Moscow State University). An active collaboration with the
Zwicky Transient Facility (ZTF) team facilitates timely classification of
detected transients and their optical follow-up on Palomar and Keck
telescopes. During the period from June 10 to December 14, 2020, spanned by
the second SRG/eROSITA all-sky survey, 16 TDEs have already been detected and
optically confirmed in the $0<l<180^{\circ}$ hemisphere (Sazonov et al. 2021,
Gilfanov et al., in preparation). The first eROSITA TDEs have also been
reported in the other half of the sky (Liu et al. 2021).

The SRG scanning strategy means that more rapid transients may be found that
vary on a timescale of about one day, in particular, X-ray afterglows of
gamma-ray bursts even without an observable gamma-ray trigger (Khabibullin et
al. 2012). A systematic search for these events is continuously ongoing (e.g.,
Wilms et al. 2020).

##  5 SRG spacecraft

Design of the structure and configuration of the Spektr-RG spacecraft
continued until 2008 (see §8 below about the history of the SRG project). At
that time, the concept design was defined and accepted.

###  5.1 Navigator-SRG platform

The spacecraft (Figs. 2, 3, and 8) is based on the Navigator-SRG space
platform (Fig. 34), which has been developed by NPOL for spacecraft of hydro-
meteorological and scientific designation. By the time the SRG design was
accepted, the flight models of the Navigator platform for the Spektr-R
(Radioastron) and Electro-L No. 1 spacecraft had been produced and tested. In
2011 the platform started its flight qualification. This meant an opportunity
to gather invaluable experience of the platform operation, which was
considered during the creation of Spektr-RG. Modifications aimed at enhancing
the mission reliability and achieving the required technical parameters of the
spacecraft as a whole were made. A principal change made to the Navigator
platform was the installation of a new X-band radiocomplex, induced by the
necessity of maintaining ground contact with the spacecraft at large
distances.

Figure 34: Navigator platform. Credit: NPO Lavochkin, see also
http://www.russianspaceweb.com/navigator.html (credit: A.Zak).

The Navigator-SRG platform includes the following units: an onboard control
system, an onboard radiocomplex, solar panels with the orientation mechanism,
a propulsion unit, a thermal control system, a telemetry system, an antenna-
feeder system, a cable network, construction, a power supply system, and
miscellaneous supplementary systems and units.

The onboard control system performs the following basic tasks: it controls
operation of the scientific payload and service systems of the spacecraft, it
controls the motion of the spacecraft around the center of mass and the motion
of the center of mass, it controls the spacecraft orientation by choosing the
correct omnidirectional antenna for linkage with ground stations, it controls
the technical state and diagnostics of onboard systems with subsequent
transfer of information to Earth via the onboard radiocomplex, it manages
contingency cases, and performs automatic transition of the spacecraft into
stand-by mode in contingency cases that cannot be managed by the onboard
systems.

The onboard control system includes the following elements: an onboard
computer, power automation units, a gyroscopic angular velocity sensor, two
SDP-1 solar position sensors, two POS 347K solar orientation devices, two
SED26 star trackers, and four Agat-15M reaction wheel sets.

The main orientation control actuators of the onboard control system are
reaction wheels (RWs). Three RWs are in operation, and one RW is in cold
reserve. A set of three RWs ensures an angular velocity up to 0.07 deg/s. For
RWs desaturation (momentum dump), 16 stabilization thrusters are used (8
operating and 8 in cold reserve, 0.5 N each). These thrusters are also used
during the initial flight phase to establish the solar orientation and in
contingency cases.

In addition to the stabilization thrusters, the propulsion unit also includes
larger correction thrusters (5 N each) that are used for trajectory
correction. All the thrusters are powered by hydrazine contained in two tanks,
each of which is filled with 180 kg of fuel.

Electric power supply for the onboard service systems and scientific
instruments is provided by solar panels. The total power consumption of all
onboard systems does not exceed 1700 W, which is less than the expected
capacity of the solar panels at the end of their service lifetime (1870 W).
The power supply system also includes a 55 Ah battery that provides power for
onboard systems during the initial phase after separation of the spacecraft
from the launch vehicle or in contingency cases.

Thermal control of the spacecraft is established by axial and contour heat
pipes, as well as by electrical heaters that are controlled by the onboard
control system. In addition, both telescopes have their own thermal control
systems.

The eROSITA and ART-XC telescopes are placed on a geometrically stable frame
mounted on the top joint of the spacecraft base module. A number of electronic
units of ART-XC and the gyroscopic angular velocity sensor device of the
onboard control system are installed on a thermo-stabilized platform.

###  5.2 Orientation and operation regimes of the observatory

Except during the initial flight phase, the onboard control system operates in
inertial orientation mode, which ensures triple-axis stabilization and
spacecraft rotation into a programmed orientation with respect to the inertial
frame. Orientation data from the star trackers are continuously coupled with
the gyroscopic sensor data, resulting in an estimated orientation quaternion.

In the inertial orientation mode, the following operations can be performed:
spacecraft rotation for pointing telescopes (SRG +X axis) to a predefined
target on the celestial sphere and triple-axis stabilization with respect to
the inertial frame; spacecraft rotation at a constant angular velocity
(typically 0.025 deg/s) around the SRG +Z axis, in which the rotation axis is
constantly rotated at a low angular velocity ($\sim 1$ deg/day) to ensure a
smooth all-sky survey; and spacecraft triple-axis stabilization during
trajectory-correction impulses.

Several consequent spacecraft rotations can be combined into series to scan
fields on a celestial sphere by predefined routes. In this mode, the onboard
control system points the SRG X-axis to the required starting point of a scan
field (with respect to the inertial frame) and performs a program of relative
rotations around the Y- and Z-axes of the spacecraft. During the relative
rotations, the coupling of the star trackers and gyro data does not interrupt
the precise route tracking with respect to the inertial frame. The scan field
size can be up to $12.5^{\circ}\times 12.5^{\circ}$. A typical scan sequence
consists of several S-turns (Fig. 35). The main scanning turns are performed
around the Z-axis and smaller auxiliary turns (steps) are performed around the
Y-axis. Step rotation can be arbitrarily low and is usually chosen from the
4–12 arcmin range. The highest angular velocity of the scanning turns may be
as high as 0.04 deg/s.

Figure 35: Typical route of the X-axis in scan mode.

The spacecraft attitude-control error has been confirmed to be less than 10
arcsec. The angular velocity stabilization error does not exceed 0.72
arcsec/s. Figure 36 illustrates the main orientation restrictions on pointing
and scanning observations of the SRG observatory.

Figure 36: Main orientation restrictions for observations during scans and
pointings.

###  5.3 Radiocomplex and data transmission

In addition to receiving commands and performing trajectory measurements, the
onboard radiocomplex is also capable of combining scientific data and service
telemetry into one data stream. The nominal rate of transmission of service
telemetry data is 16 Kbit/s (real-time transmission of TM-data) and 32 Kbit/s
(stored TM-data dump).

The nominal rate of scientific data transmission is 512 Kbit/s, but it can
also be performed at 64, 128, and 256 Kbit/s. Telecommands from ground
stations are uploaded at a rate of 500 bits/s (typical) or 125 bits/s
(contingency cases).

The radiocomplex receives telecommands through an omnidirectional antenna. It
transfers data through a medium-gain antenna with a beam width of $\pm
24^{\circ}$.

The onboard telemetry system collects service telemetry sensor data and
digital telemetry data from the onboard control system and writes it into the
internal memory device. During ground contact, it is possible to dump the
stored telemetry data or run live telemetry transfer mode.

###  5.4 Launch and operational orbit insertion

Table 2: Trajectory-correction schedule for the SRG flight to the vicinity of the L2 point. No. | Date/time | Days in flight | Number of PU ignitions | Total characteristic speed | Consumed propellant
---|---|---|---|---|---
| (Moscow) |  |  | m/s | kg
1 | Jul. 22, 2019 | 10 | 2 | 13.59 | 15.98
| 17:30:00.000 |  |  |  |
2 | Aug. 06, 2019 | 25 | 2 | 3.49 | 4.09
| 17:30:00.000 |  |  |  |

The SRG observatory operates in a quasi-periodic orbit around the L2 libration
point of the Sun–Earth system. This orbit has a number of clear advantages for
the mission related to the remoteness (1.5 million km, corresponding to a
light travel time of 5 s) of the L2 point from Earth in the direction away
from the Sun, in particular, long visibility periods from ground stations for
control and receipt of scientific data, and the prevention of periodical
entries of the spacecraft into Earth’s radiation belts.

The location of the Russian ground-control stations in the northern hemisphere
required choosing a launch date that would ensure maximum daily radio
visibility of the spacecraft. In addition, the choice of an optimal
operational orbit had to minimize the characteristic velocity required to keep
the spacecraft near the L2 point. The formal solution based on these two
criteria implied the necessity of preparing separate flight plans for the
upper stage for each possible launch date.

Taking into account the anticipated time frame of the readiness of the
spacecraft segments, two possible launch windows were eventually chosen: June
21-22, 2019, and July 12-13, 2019. The launch of the SRG spacecraft from the
Baikonur Cosmodrome successfully occurred on July 13, 2019, at 15:30:57 Moscow
time.

The Proton-M launch vehicle placed the orbital unit (OU), including the upper
stage (US), the adapter system, and the spacecraft, in a falling trajectory
with an apogee altitude of $\sim 200$ km. The first ignition of the propulsion
unit (PU) of the US placed it in a transfer orbit $\sim 168\times 2013$ km.
The second ignition placed the OU in the nominal flight orbit toward the L2
point $\sim 500\times 1450000$ km. The spacecraft later separated from the US,
followed by the deorbiting of the US. The placement into orbit took two hours.
During this process, the US spent 34 minutes in the Earth’s shadow. The flight
toward L2 lasted approximately 100 days without any shadow intervals.

To ensure placement of the spacecraft in the nominal orbit, three flight
trajectory corrections on the way to the L2 point had been planned: on days
10, 20, and 40 of the mission. The corresponding reserve dates were also
defined: on days 15, 25, and 45. A characteristic velocity budget of 100 m/s
was allocated for these corrections. The first correction was executed in
accordance with the schedule. The second correction was moved to the reserve
date to acquire more orbit measurements after the first correction. By the
time the third correction was due, the calculations indicated that it was not
needed, so that there was no correction on day 40 (see Table 2).

The operational orbit of the SRG observatory was defined by the following
constraints: the maximum distance of the spacecraft from the L2 point in the
ecliptic plane $\sim 920000$ km, the exit from the ecliptic plane $\sim
700000$ km and $\sim 550000$ km toward the north and south ecliptic poles,
respectively. After placement of the spacecraft into the nominal orbit near
the L2 point (on day 100), orbit corrections are executed approximately every
50 days during the mission (see §5.7 below).

Figure 37: Minimum visibility without corrections. Figure 38: Minimum
visibility with large corrections.

###  5.5 Spacecraft control

Spacecraft operation planning is performed on three timescales (long term,
medium term, and short term). The degree of detail is to be increased when
proceeding to the next stage.

Long-term mission planning is determined by science tasks formulated by the
scientific community. At the medium-term level, operations of the spacecraft
and ground stations are planned for the following calendar month.
Approximately two weeks before its initiation, NPOL performs a priori
calculations of the spacecraft trajectory for this month, schedules
preliminary intervals for spacecraft control sessions, and, if necessary,
schedules technological operations for the spacecraft (which usually include
only trajectory correction). NPOL delivers these data to the science operation
and data center of IKI, where the month plan is completed with scientific
operations in accordance with the long-term plan. The plan produced at this
stage determines the sequence and parameters of the spacecraft orientation
modes. The plan undergoes preliminary verification of consistency with the
spacecraft and ground-stations operation limitations both on the side of IKI
and NPOL. The spacecraft orientation limitations are the main verification
criteria. In case of changes in the ground stations or in the onboard systems’
condition, a monthly plan can be adjusted even during its implementation. In
these cases, secondary verification and decision making usually takes no
longer than three days.

At the short-term level, exact operations of the ground station and spacecraft
during an upcoming ground-contact session are planned. The main task of short-
term planning is to ensure necessary spacecraft operations starting from the
beginning of a session until at least the next session. Operations scheduled
in a month plan are detailed in the form of a ground-contact session program,
which is a sequence of commands for ground stations, spacecraft service
systems, and the telescopes. To compose a ground-contact session program, NPOL
collects requests for commands from supervisors of the corresponding systems.
In particular, a sequence of commands to ensure the telescopes’ operation
(with specific command parameters and issue times or intervals) is generated
at IKI based on requests of the ART-XC and eROSITA teams.

Commands for the spacecraft and scientific instruments may be executed
immediately during a ground-contact session or stored in the onboard control
system memory for execution at a preset time (time-tagged commands). NPOL
performs a simulation of a prepared ground-contact session on the onboard
control system’s logical testing bench. It provides means of precise
verification of a program consistency with spacecraft operation limitations,
and detects logical planning errors and incorrect commands. Simulation is
performed for an interval from the start of a planned control session until
the start of the next session. This technology ensures end-to-end simulation
during the whole mission, which increases control reliability.

Provided the simulation results are positive and all the parties endorse a
control session plan, it is executed. The execution of the commands is
controlled using telemetry data. If necessary, the program may be adjusted
during a session, including issuance of commands initially not planned by the
ART-XC and eROSITA teams.

Ground-contact sessions take place daily. Each session lasts approximately 4–5
hours, which makes it possible to dump all scientific and service telemetry
data gathered throughout the day, conduct trajectory measurements by at least
two ground stations, check the status of onboard systems and scientific
instruments, and upload a time-tagged command sequence for a subsequent period
of up to several days. During a control session, the full real-time data
stream from the spacecraft is received by NPOL through ground stations and
transmitted to IKI.

###  5.6 Mission ground-control complex

Spacecraft control is provided by the Russian ground-control complex of the
SRG spacecraft, including the NPOL mission control center, ground stations,
ballistic centers, and means of communication. The central authority
responsible for the flight tests and spacecraft control is NPOL, which
provides a platform for the cooperation of specialists participating in flight
tests and the implementation of the scientific program.

Radio communication with the spacecraft is provided by radio-technical ground
complexes located at Baikonur (TNA-57 antenna of 12 m diameter), Bear Lakes
(TNA-1500 antenna of 64 m diameter), and Ussuriysk (P-2500 antenna of 70 m
diameter). Currently, only the Bear Lake and Ussuriysk ground complexes are
used for receiving scientific data. The radio-technical ground complexes had
been modernized for the implementation of the SRG project and currently fully
ensure the mission requirements. During periods of short daily radiovisibility
intervals from the Russian Deep Space Network antennas (which usually happen
in April and May), the Malargue, Cebreros, and New Norcia stations of the
ESTRACK network are involved when necessary in receiving scientific data,
according to the agreement between Roskosmos and ESA.

Ballistic maintenance of the mission is provided by two ballistic centers
located at the Keldysh Institute of Applied Mathematics of the Russian Academy
of Sciences and the Central Research Institute of Machine Building
(TsNIIMash). All elements of the ground-control complex are united into one
data system by means of a high-bandwidth data network, which ensures data
exchange between the elements of the ground-control complex and the scientific
ground complex.

###  5.7 Large corrections of the SRG orbit

Table 3: Trajectory-correction schedule for the orbiting of SRG near the L2 point. No. | Date/time | Days in flight | Number of PU ignitions | Total characteristic speed | Consumed propellant
---|---|---|---|---|---
| (Moscow) |  |  | m/s | kg
1 | Oct. 21, 2019 | 100 | 1 | 0.21 | 0.25
| 19:00:00.000 |  |  |  |
2 | Dec. 10, 2019 | 150 | 1 | 0.18 | 0.21
| 19:00:00.000 |  |  |  |
3 | Jan. 30, 2020 | 201 | 1 | 0.27 | 0.32
| 19:00:00.000 |  |  |  |
4 | Apr. 01,2020 | 263 | 1 | 1.00 | 1.13
| 19:00:00.000 |  |  |  |
5 | Jun. 16, 2020 | 339 | 1 | 0.95 | 1.07
| 21:00:00.000 |  |  |  |
6 | Aug. 05, 2020 | 389 | 1 | 0.94 | 1.08
| 20:00:00.000 |  |  |  |
7 | Oct. 05, 2020 | 450 | 1 | 3.01 | 3.38
| 19:00:00.000 |  |  |  |
8 | Nov. 23, 2020 | 499 | 1 | 6.23 | 6.99
| 19:00:00.000 |  |  |  |
9 | Feb. 28, 2020 | 588 | 1 | 6.24 | 6.94
| 17:00:00.000 |  |  |  |
Remaining propellant: 319.63 kg

After placement of the spacecraft into the nominal orbit near the L2 point (on
day 100), station keeping corrections were executed approximately every 50
days until September 2020 (see Table 2). Because the launch date was chosen
using many criteria, there is a gap of about a month every Spring in the
radiovisibility of SRG in its nominal operational orbit from the Russian
ground-control stations (see Fig. 37). In the Fall of 2020, a strategy of
large manoeuvres began to be implemented (see Table 3) to alleviate this
problem. This strategy was developed in accordance with Canalias & Masdemont
(2004) and consists of one test manoeuvre (3 m/s) and ten manoeuvres (6 m/s
each). As demonstrated by calculations (see Fig. 38), it should enhance the
visibility of SRG from the Russian Deep Space Network antennas during Spring
in the next years. This will significantly improve the implementation of
control operations with the Navigator platform and telescopes as well as the
dumping of science data.

##  6 Mikhail Pavlinsky ART-XC

ART-XC on board the SRG spacecraft is an X-ray grazing-incidence mirror
telescope array. It was developed by the Space Research Institute (IKI) and
the All-Russian Scientific Research Institute for Experimental Physics
(VNIIEF). The NASA Marshall Space Flight Center (MSFC) produced the flight
modules of the X-ray mirror systems. ART-XC is designed for conducting an all-
sky survey in the 4–12 keV energy band and pointed observations of selected
astrophysical objects in the 4–30 keV energy band.

###  6.1 Design

ART-XC consists of seven identical mirror systems (MS) paired with the unit of
the Rontgen detector (URD) (Fig. 39). Each MS plus URD pair forms a telescope.
All seven of them are pointing in the same direction.

Figure 39: Internal arrangement of the ART-XC telescope: Cone-shaped carbon
fiber tube with seven identical X-ray mirror systems at the top. The mirror
systems focus X-rays onto seven detector units.

The basic structure of ART-XC is a cone-shaped carbon fiber tube with a height
of three meters. The MSs are mounted on top of this tube and focus X-ray
photons onto their respective URDs. A sunshield protects the MSs from direct
sunlight. The upper part of this tube is covered with copper shielding to
reduce the stray-light background in the detectors. Each URD is equipped with
a collimator to reduce stray-light background. The collimator includes a block
of calibration X-ray sources (55Fe + 241Am) for in-flight calibration. Heat
pipes and a radiator are used to maintain the operating temperature of the
detector at about $-20\deg$C. The star tracker is mounted near the MSs.

Figure 40: Wolter-I optical scheme of ART-XC. $Dfr_{28}$ and $Dfr_{1}$ are the
entrance apertures of the outermost and innermost shells, respectively,
$l_{h}$ and $l_{p}$ are the heights of the paraboloids and hyperboloids,
respectively, and $f_{W}$ is the focal length of the mirror system. Adapted
from Pavlinsky et al. (2020).

The ART-XC MSs were produced and calibrated by MSFC (Gubarev et al. 2012,
2014; Krivonos et al. 2017). Each MS contains 28 Wolter-I (Fig. 40) nested-
mirror shells. The shells were fabricated using an electroformed-nickel-
replication technique and coated with a $\sim 10$ nm layer of 90% bulk density
iridium. The shell thickness varies slightly with radius: the thickness of the
outer shells is larger than the nominal thickness to make them stiffer and
thus to improve the angular resolution of the module. The upper ends of the
shells are glued to the supporting spider. The nominal focal length of the MSs
is 2700 mm. They were defocused by 7 mm during installation on the telescope
to provide a more uniform angular resolution across the FoV.

The detector system of ART-XC consists of seven URDs, two electronic modules,
and a serial interface connection module. The position-sensitive X-ray
detector for ART-XC was developed by IKI (Levin et al. 2014, 2016). The
sensitive element is a double-sided strip detector (DSSD) based on a CdTe die
with dimensions of $29.953\times 29.953\times 1$ mm3. The high-quality CdTe
dies were manufactured specifically for IKI by Acrorad Co. Ltd. (Japan).
Coordinate resolution is provided by two mutually perpendicular sets of 48
strips on the two sides of the crystal. For signal acquisition, two VA64TA1
ASICs per detector are used, one on each side. The ASICs were manufactured by
Ideas (Norway). More than 30 URDs were produced and tested at IKI. Seven of
them were installed in the flight model of the ART-XC telescope.

###  6.2 Characteristics

Figure 41: ART-XC effective area for doubly reflected events (blue) and singly
reflected events (red) at 8.1 keV as a function of offset angle (left). ART-XC
FoV-averaged effective area for doubly (blue) and singly reflected events
(red) in the 4–35 keV energy band as a function of energy (right). The graphs
for the effective area are based on the effective area of the simulated mirror
system and on the efficiency of the spare detector.

The ART-XC FoV is $\sim 36$ arcmin ($\sim 0.3$ sq. deg), within which the MSs
provide an angular resolution better than 1′. The effective area (on-axis) is
substantial up to energies $\sim 30$ keV. In addition, there are photons that
are only once reflected from the MSs, and as a result, they can fall onto the
detector with offset angles up to $\sim 50^{\prime}$. Figure 41 illustrates
the effect of these singly reflected photons on the ART-XC FoV and their
contribution to the FoV-averaged effective area. For this type of events, the
ART-XC FoV is $\sim 2$ square degrees. Although true imaging cannot be done in
this extended FoV and singly reflected photons generally cause an increase in
the background, they can be used to measure the X-ray fluxes of bright
sources. Therefore, the ART-XC telescope can also be exploited as a
concentrator. In survey mode, ART-XC can therefore monitor bright transient
sources for at least 28–32 hours (Pavlinsky et al. 2019a).

The URD can register photons with energies up to $\sim 100$ keV, with an
energy resolution of 9% at 13.9 keV. The detector strip size corresponds to an
angular size of 45′′. During a calibration campaign at the IKI X-ray test
facility (Pavlinsky et al. 2018, 2019b, 2019a), it was determined that the
efficiency of the ART-XC spare URD reaches 50% in the 4.47–4.76 keV energy
range and 90% in the 9.43–10.04 keV energy range.

Based on the model for the MS effective area and on the telescope ground
calibrations, the on-axis effective area, vignetting, and grasp of ART-XC have
been estimated. The ART-XC on-axis effective area at 8.1 keV is 385 cm2. The
grasp at 8.1 keV is 43.8 cm2 deg2. Table 4 summarizes the performance
parameters of the ART-XC.

Table 4: ART-XC characteristics.

Telescope mass 350 kg Dimensions 3.5 m $\times\diameter 0.9$ m Power 150 watts
Number of modules 7 Nominal focal length 2700 mm Operational energy range 4–30
keV FoV 0.3 sq. deg Eff. area for pointed observations 385 cm2 @ 8.1 keV Grasp
43.8 cm2 deg2 @ 8.1 keV Ang. resolution (FWHM) in survey
$53\hbox{${}^{\prime\prime}$}$ Energy resolution 9% @ 13.9 keV Time resolution
23 $\mu$s

###  6.3 Scientific goals

The main goal of ART-XC is to survey the whole sky in the broad X-ray energy
range of 4–30 keV with a sensitivity of $\sim 10^{-12}$ erg s-1 cm-2 ($\sim
10^{-13}$ erg s-1 cm-2 near the ecliptic poles) in the 4–12 keV band and an
angular resolution better than an arcminute. The ART-XC survey will thus be
the most sensitive all-sky survey ever conducted at these energies. The ART-XC
survey complements the all-sky survey performed by eROSITA in the overlapping
energy band of 0.3–8 keV (with the highest sensitivity below 2 keV). The
harder energy band of the ART-XC survey is particularly valuable for studying
populations of heavily obscured astrophysical objects.

Preliminary estimates show that during the four-year all-sky survey, ART-XC
will detect $\sim 5000$ X-ray sources, mostly AGN, including heavily obscured
ones (with hydrogen column densities $N_{\rm H}\gtrsim 10^{23}$ cm-2). This
will provide a rich database for explorations of the AGN population at
$z\lesssim 0.3$.

ART-XC will also provide valuable information about the temperature of the
intracluster gas in rich low-redshift clusters of galaxies. This will help
tighten constraints on the cosmological parameters inferred from the eROSITA
all-sky survey.

The ART-XC all-sky survey can make breakthroughs in studying various classes
of Galactic X-ray sources, such as X-ray binaries and cataclysmic variables
(CVs). As many as $\sim 10^{3}$ CVs can be found, compared to $\lesssim 100$
known from previous X-ray surveys.

With its unique combination of broad energy coverage, good angular resolution,
and wide FoV, ART-XC should make major advances in the study of SNRs and the
Galactic Ridge X-ray emission.

Finally, ART-XC is well suited for discovering and monitoring transient and
variable X-ray sources, such as X-ray and gamma-ray bursts, Galactic X-ray
transients, and AGN. Although the probability of catching GRBs in the ART-XC
FoV is not high, bright GRBs can penetrate the shielding material of the
telescope and induce a signal on the detectors. For these events, ART-XC can
provide precise timing information that can be used to localize them through
triangulation with other space observatories.

###  6.4 In-flight performance

ART-XC has been operating in orbit for almost two years at the time of
writing. The results obtained during the CalPV phase and the all-sky survey so
far fully confirm the expected unique capabilities of the instrument.

In particular, in order to calibrate the ART-XC effective area, a series of
observations of the Crab nebula was performed during the CalPV phase. The
effective area proved to agree well with the results of simulations and ground
calibrations (Pavlinsky et al. 2018, 2019b, 2019a).

##  7 eROSITA

###  7.1 History

The concept of the eROSITA telescope is based on a long series of previous
scientific and technological developments dating back to the very successful
German, US, and UK ROSAT mission (1990-1999; Trümper 1982), which was
developed and managed under the leadership of the Max Planck Institute for
Extraterrestrial Physics (MPE). ROSAT carried out the first complete survey of
the sky with an imaging X-ray telescope in the energy range between 0.1 and
2.4 keV, and it performed tens of thousands of pointed observations.

The following generation of large X-ray telescopes, the NASA-led Chandra
Observatory and the ESA mission XMM-Newton, required the development of mirror
systems with longer focal length (7.5-10 meters) in order to bring harder
X-ray radiation into focus. These observatories, both launched in 1999, were
only able to perform pointed observations, however, their limited FoV meant
that it was not possible to perform large-area surveys. This led to the
proposal of a hard X-ray imaging telescope with all-sky surveying
capabilities. The Astrophysikalisches Institut Potsdam (AIP), the MPE, and the
University of Tübingen (IAAT) therefore proposed the mission called A BRoad-
band Imaging X-ray All-sky Survey (ABRIXAS).

From the very beginning, the ABRIXAS mission concept was developed through the
coherent adaptation of mirror and detector technologies developed for XMM-
Newton for a national, small-scale satellite mission. Consequently, the
development timescale for the project was relatively short (about three years)
and the overall costs moderate. The seven mirror modules shared in the focal
plane an identical copy of the pn-CCD camera developed for XMM-Newton, and
with their small focal length of 1.6 meters, they were ideal for a small
satellite. Because of a design failure in the power supply, however, the
satellite lost its main battery soon after launch, in April 1999, and could
never be used for scientific purposes. The hard- and software developments
enabled by the ABRIXAS mission have been extremely useful in the framework of
a number of subsequent projects, however.

Figure 42: Pictures of the fully integrated eROSITA telescope taken during the
final space qualification campaign in December 2016. Front view of the
telescope (with open cover) with all seven mirror assemblies installed (left).
Rear view of the telescope with all seven camera assemblies installed (right).

Despite the unfortunate outcome of the ABRIXAS mission, the appeal of the
original scientific goal of an imaging hard X-ray all-sky survey remained
high, with no other planned astrophysical mission with similar objectives on
the horizon. With the help of the Semiconductor Laboratory (HLL) for the
production of high-sensitivity detectors of the MPE, a new project to further
develop the highly successful XMM-Newton pn-CCD technology was launched. In
March 2002, all participating institutes proposed to ESA to accommodate the
ROentgen Survey with an Imaging Telescope Array (ROSITA) telescope on an
external platform of the International Space Station (ISS). The seven mirror
modules would have been built identically to those of ABRIXAS, but each of
them would have been equipped with its own newly developed, frame-store pn-CCD
in the focal plane. In September 2002, this proposal was supported by ESA with
the highest scientific priority, and was recommended for phase A study. Launch
would only have been possible from 2011 onward, however, because the external
platforms of the ISS was occupied before then. Subsequently, it emerged that
the ISS would not have been a viable option for ROSITA, first of all because
NASA decided to terminate its shuttle flights to the ISS in 2010, and
furthermore, because a contamination experiment on the ISS revealed that its
environment was not safe from contamination for the sensitive X-ray mirrors
and detectors of ROSITA.

At the turn of the millennium, the observations of supernovae type Ia by two
independent groups (awarded with the Nobel Prize for Physics in 2011 for their
discoveries) revealed that the expansion of the Universe is accelerating, a
fact that may suggest the existence of a cosmological constant. The subsequent
measures, taken by Boomerang and WMAP, of the tiny temperature fluctuations of
the microwave background radiation out of which galaxies, clusters of
galaxies, and the overall large-scale structure of the Universe originated,
pointed toward a flat Universe, whereby the total matter plus energy content
of the cosmos attains almost exactly the critical value. In fact, studies of
the baryon fraction in X-ray selected (mainly from ROSAT) clusters of galaxies
throughout the 1990s had already shown compelling evidence that the matter
density was lower than unity (Schuecker et al. 2003).

The fact that a very large sample of clusters of galaxies can be particularly
useful for precision cosmology stimulated many different groups to conceive
dedicated large-area cluster surveys. In April 2003, members of the ROSITA
team participated in the proposal of DUO, a NASA SMEX-mission, based on a
modification of the ROSITA telescope design. Together with another four
missions, DUO was selected among 36 competing proposals for a phase-A study,
carried out in 2004. DUO was to survey an area of the sky of about 6,000 deg2,
overlapping that explored by the optical Sloan Digital Sky Survey (SDSS). In
this way, about 10,000 clusters of galaxies could have been detected,
providing constraints on the dark energy density to an accuracy of within 10%.
NASA did not select the DUO-project for further development, however, and the
only mission of the five that was finally executed within the SMEX program was
the hard X-ray focusing telescope NuSTAR, which was launched in 2012.

Table 5: Basic eROSITA instrument parameters in launch configuration Instrument | 7 Mirror Assemblies | 7 Camera Assemblies
---|---|---
Size | 1.9 $\diameter$ $\times$ 3.5 m | Diam. of outer shell | 358 mm | CCD image |  $2.88\times 2.88$ cm2 [$1\aas@@fstack{\circ}{}03\times 1\aas@@fstack{\circ}{}03$]
Mass | 808 kg | Number of shells | 54 | pixelsize |  $75\mu\mathrm{m}\times 75\mu\mathrm{m}$ [$9\aas@@fstack{\prime\prime}{}6\times 9\aas@@fstack{\prime\prime}{}6$]
Power | 522 W max. | focal length | 1600 mm | Time Resol. | 50 msec
Datarate | 600 MB/day max. | HEW on axis (1.5 keV) | $18^{\prime\prime}$ | Energy Resol | 70 eV [1 keV]
|  | HEW FoV averaged | $26^{\prime\prime}$ | Qu-Efficiency | 95%

In February 2005, the Astronomy and Astrophysics Advisory Committee (AAAC),
founded by the National Science Foundation (NSF), NASA, and the Department of
Energy (DOE), and the High Energy Physics Advisory Panel (HEPAP), founded by
NSF and DOE, established a Dark Energy Task Force (DETF) with the task of
advising NSF, NASA, and DOE on the optimal strategies for the future of dark
energy research. In particular, the DETF evaluated and compared various
ground- and space-based instrumentation and observational methods. A dedicated
white paper (Haiman et al. 2005) reached the conclusion that within available
technologies at the time, it would have been possible to gather a sample of
about 100,000 X-ray selected clusters of galaxies that would have provided
very stringent constraints on the fundamental parameters of the cosmological
model of the Universe. The construction of a sample of galaxy clusters of this
magnitude is the primary objective of the eROSITA telescope. To achieve this
goal, the ABRIXAS mirror design had to be modified to include 27 additional
shells, doubling the diameter of each telescope module and increasing the
effective area at low energies by a factor of five. With this fundamental
rescope, eROSITA will likely be the first stage IV dark energy probe to be
realized and will outperform the DUO-like stage IV probe originally considered
by the DETF.

In June 2006, the funding proposal for eROSITA was submitted to the German
Space Agency (DLR). Five German institutes (MPE, IAAT, AIP, the Hamburg
Observatory, and the Dr. Remeis-Sternwarte in Bamberg, the Astronomical
Institute of the Erlangen-Nürnberg University) agreed to work together to
develop, build, and organize the scientific exploitation of the instrument and
formed the German eROSITA Consortium. They were later joined by three more
institutes: the Argelander-Institut für Astronomie at the University of Bonn,
the Max-Planck-Institut für Astrophysik (MPA), and the Universitäts-Sternwarte
München (LMU). In March 2007, DLR approved the project and funded the eROSITA
telescope, while a Memorandum of Understanding was signed between DLR and the
Russian Space Agency (Roskosmos) to ensure that eROSITA would be launched in
the framework of the Spektrum-Roentgen-Gamma mission. Soon afterward, the
construction of the telescope began. In September 2008, Roskosmos came to a
final decision on the payload orbit and launcher. eROSITA would be launched by
a Zenit-Fregat launcher (later in 2016 to be replaced by a Proton-M and Block-
DM combination) together with the Russian hard-X-ray telescope ART-XC on an L2
orbit to ensure maximum efficiency for a survey instrument. Eventually,
additional funds from the Max Planck Society and DLR were secured for eROSITA
in July 2009, to finance the cost-driving L2 orbit. A detailed agreement
between Roskosmos and DLR was signed the following month.

In the following years, the teams in Germany and Russia were intensively kept
busy in the development and construction of the instrument. The mirror
modules, smaller versions of the XMM-Newton mirrors, were supposed to be built
using the same technology and by the same industrial consortium. However,
there were significant problems with the shorter focal length and the smaller
mirror shells, as figuring errors have a significantly greater effect on the
image quality. The mirror engineering model had only a 42 arcsec HEW, almost
three times lower than required. Fixing these issues took more development
than expected, but was ultimately successful. Another major challenge was the
development of the electronics: after the decision for the L2 orbit, the
basically completed design had to be changed to use radiation hard components.
All qualification tests were carried out in the MPE test facilities or in the
facilities of companies in the Munich area.

eROSITA was finally delivered to Russia in January 2017. A series of tests was
then conducted at NPOL before integration of the instruments into SRG could
begin. This was then completed in April 2019 with the transport to the
Baikonur Cosmodrome.

Figure 43: eROSITA 0.3–2.3 keV wavelet-filtered X-ray image of the Abell
3391/95 system, also showing several clumps of diffuse gas at the same
distance as well as several background galaxy clusters. One degree corresponds
to 3.9 Mpc at the redshift of A3391. From Reiprich et al. (2021).

###  7.2 Instrument

eROSITA (Fig. 42) consists of seven identical and coaligned X-ray telescopes
housed in a common optical bench in a hexagonal shape. A system of carbon
fibre honeycomb panels connects the seven mirror assemblies on the front side
with the associated seven camera assemblies on the focal plane side. The
optical bench is connected to the S/C bus through a hexapod structure.

Each of the mirrors comprises 54 paraboloid or hyperboloid mirror shells in a
Wolter-I geometry, with an outer diameter of 360 mm and a common focal length
of 1 600 mm (Friedrich et al. 2008; Arcangeli et al. 2017). The average on-
axis resolution of the seven mirror modules as measured during the on-ground
calibration is $16.1^{\prime\prime}$ half-energy width (HEW) at 1.5 keV. The
unavoidable off-axis blurring typical of Wolter-I optics is compensated for by
a 0.4 mm shift of the cameras toward the mirrors. This places each telescope
slightly out of focus and leads to a slight degradation of the on-axis
performance ($18^{\prime\prime}$), but improves the angular resolution
averaged over the FoV. Each mirror assembly has a CCD camera in its focus
(Meidinger et al. 2014). The eROSITA CCDs are advanced versions of the EPICpn
CCDs on XMM-Newton. They have $384\times 384$ pixels in an image area of
$28.8\,\mathrm{mm}\times 28.8\,\mathrm{mm}$, yielding a square FoV of
$1\aas@@fstack{\circ}{}03\times 1\aas@@fstack{\circ}{}03$. Each pixel
corresponds to a sky area of $9\aas@@fstack{\prime\prime}{}6\times
9\aas@@fstack{\prime\prime}{}6$. The nominal integration time for all eROSITA
CCDs is 50 msec. The additional presence of a frame-store area in the CCD
substantially reduces the amount of so-called out-of-time events, which are
recorded during the CCD read-out, a significant improvement with respect to
the PN camera on XMM-Newton (Strüder et al. 2001). For optimal performance
during operations, the CCDs are cooled down to about $-85^{\circ}$ by means of
passive elements (Fürmetz et al. 2008).

For calibration purposes, each camera has its own filter wheel with a
radioactive 55Fe source and an aluminum–titanium target. This provides three
spectral lines at 5.9 keV (Mn-K$\alpha$), 4.5 keV (Ti-K$\alpha$), and 1.5 keV
(Al K$\alpha$).

The electronics for onboard-processing of the camera data is provided by seven
sets of camera electronics (CE), each one mounted and interfaced to the
cameras. Each of the CEs provides the proper voltage control and readout
timing of the associated camera and performs the onboard data processing
within the time constraints of the camera integration time.

Figure 44: False-colour images of the the first eROSITA all-sky survey
(eRASS1, left) and the second eROSITA all-sky survey (eRASS2, right)
observations in the energy bands 0.2–0.6 keV (red), 0.6–1.0 keV (green),
1.0–2.3 keV (blue), adaptively smoothed. The size of the images is 3 degrees
$\times$ 3 degrees. The circular structure is a dust scattering ring, echoing
the outburst of the X-ray binary MAXI J1348-630. From (Lamer et al. 2021).

The Interface and Thermal Controller (ITC) receives the telemetry generated by
each CE and stores it in the mass memory, commands each of the CEs, and
controls the power distribution and the temperatures of the mirrors and
cameras. Given its criticality, the ITC is a cold redundant unit (Coutinho et
al. 2018).

Finally, two (redundant) star trackers are mounted on eROSITA for the
determination of an accurate boresight. The dimensions of the telescope
structure are approximately 1.9 m (diameter) $\times$ 3.2 m (height in launch
configuration, with closed front cover). The total weight of eROSITA is 808
kg. Table 5 summarizes the basic eROSITA instrument parameters.

Table 6: Summary of performance characteristics of the eROSITA telescope and its survey sensitivity. The background counts are based on the first all-sky survey data. For eRASS:1 and the PV eFEDS 140 deg2 survey (Brunner et al. 2021), the flux sensitivity in each band has been computed by taking all sources detected above a likelihood of 8 (soft band) or 10 (hard band), and measuring the flux below which the logarithmic number counts start declining. For eRASS:8 the predictions are based on detailed simulations that include all instrumental effects and particle background intensity consistent with that measured at L2. For each field or region, we quote the total (unvignetted) exposure in seconds. The corresponding effective (vignetted) exposures can be computed by dividing the total exposure by 1.8 and 3.31 for the soft and hard band, respectively.  | Energy Range
---|---
| Soft Band | Hard Band
| 0.2–2.3 keV | 2.3–8 keV
FoV averaged effective area [cm2] | 1,365 @ 1keV | 139 @ 5 keV
Total Background [10-3 cts/s/arcmin2] |  $\approx$ 3.7 |  $\approx$ 2.1
Point source sensitivity eRASS:1
Ecliptic Equatorial region | Total exposure [s] = 200 |  $5\times 10^{-14}$ erg/s/cm2 |  $7\times 10^{-13}$ erg/s/cm2
Ecliptic Polar region | Total exposure [s] = 4000 |  $7\times 10^{-15}$ erg/s/cm2 |  $9\times 10^{-14}$ erg/s/cm2
Point source sensitivity eFEDS
eFEDS field | Total exposure [s] = 2500 |  $9\times 10^{-15}$ erg/s/cm2 |  $1.3\times 10^{-13}$ erg/s/cm2
Point source sensitivity eRASS:8 (predicted)
Ecliptic Equatorial region | Total/Effective exposure [s]= 1600 |  $1.1\times 10^{-14}$ erg/s/cm2 |  $2.5\times 10^{-13}$ erg/s/cm2
Ecliptic Polar region | Total exposure [s] = 30000 |  $2.5\times 10^{-15}$ erg/s/cm2 |  $4\times 10^{-14}$ erg/s/cm2

###  7.3 Early operations, first results, and performance

The commissioning phase of eROSITA had the objective of switching on all
subsystems to verify they were functional following the launch and that they
performed as expected to fulfill the scientific objectives. This phase served
not only to test and commission the complete eROSITA telescope, but also for
the teams in Khimky and Moscow (NPOL, IKI) and Garching (MPE) to learn and
update the procedures on how to safely operate the spacecraft and the
telescopes in space. The first mission-critical event was the switch-on of the
ITC, which had to happen less than four hours after launch, to enable the
thermal control of mirrors and electronics. The second event was the opening
of the telescope cover, which occurred on July 22. Camera switch-on, instead,
had to wait several days before being activated to avoid excess contamination
from the first two spacecraft burns, which occurred on days 10 and 25 of the
mission. In addition, camera cooling could not be performed without a minimum
of 21 days of out-gassing following the cover opening. The commissioning of
the cameras lasted about two months, including time necessary to perform a
series of tests of the functionality of the camera electronics and thermal
balance system.

All seven eROSITA X-ray telescope modules had been observing the sky
simultaneously since October 13. Over the following eight weeks, eROSITA
collected its first-light images and performed a series of observations
designed to accurately calibrate the instruments and verify that the
performance of the telescope met pre-launch expectations. The Russian and
German science teams jointly defined this CalPV program, which included a
combination of pointed observations, field scans, and full-circle scan tests.

Figures 43 and 44 show two examples of images captured by eROSITA during the
CalPV and early survey phase. They highlight the main properties of this
unique X-ray telescope, namely, its ability to take deep images, which are
highly sensitive to both point-like and diffuse emission, over very large
areas of the sky. As a preview of the eROSITA capabilities, a mini-survey
called eROSITA Final Equatorial Depth Survey (eFEDS) was devised as part of
the PV plan, imaging a 140 deg2 patch of the sky to a depth comparable to that
expected at the end of the all-sky survey (see Table 6). These data confirm
the sensitivity of the X-ray telescope to its main target classes with high
accuracy. This mini-survey revealed more than 20,000 point-like X-ray sources,
about 80% of which are distant AGN harboring growing supermassive black holes.
Most of the remainder are X-ray active stars.

Finally, on December 13, 2019, the all-sky survey began. It was completed on
June 12, 2020, after 182 days of almost continuous scanning of the sky. Eight
all-sky surveys are planned in total, each delivering an average exposure with
eROSITA of about $200\mathrm{s}/cos(lat)$, where $lat$ is the ecliptic
latitude, while the area of 1 square degree around the ecliptic poles is
revisited every four hours. This accumulates an exposure of about 30 ks per
survey.

During the course of the all-sky survey, the spacecraft has been rotating
continuously with a scan rate of 90 deg hr-1, giving a four-hour period per
revolution. The rotation axis is oriented to the neighborhood of the sun, with
an average progression in ecliptic longitude of about 1 deg day-1, thus
completing one all-sky survey in half a year. The scan speed guarantees that
the angular resolution is not degraded by smearing of photons during the 50 ms
CCD read-out cycle, and it provides sufficient overlap between individual
scans to enable source variability analysis and homogeneous survey exposure.

A preliminary analysis indicates that more than one million X-ray point
sources and about 20,000 extended sources are detected in the survey. This is
comparable to and may indeed exceed the total number of X-ray sources known
before eROSITA. About 80% of the point sources are distant AGN (comprising 80%
of all known blazars, among others), and 20% are coronally active stars in the
Milky Way.

In summary, during its first year of operations in space, most technical,
operational, and scientific design characteristics of the eROSITA instrument
onboard SRG have been validated. Table 6 describes the main performance
characteristics of eROSITA based on the data collected in this period during
the PV phase and the all-sky survey. Compared to the pre-launch estimates of
Merloni et al. (2012, table 4.4.1 there), the performance closely matches the
expectations in the soft energy band, while it is slightly poorer in the hard
band, mainly because the level of particle background is higher.

##  8 History of the SRG project in Russia

###  8.1 International X-ray astronomy projects in Russia

The history of international X-ray astronomy space projects in the USSR and
Russia began with the Roentgen observatory on the Kvant module of the Mir
space station. This project had been proposed by the Space Research Institute
(IKI) of the USSR Academy of Sciences for implementation in the framework of
the Soviet Intercosmos program. A number of European institutions were invited
to this project: (i) the University of Birmingham (UK) and the SRON
Netherlands Institute for Space Research, which built the TTM coded-mask-
aperture X-ray telescope, sensitive in the 2–25 keV energy band, (ii) the MPE
(Germany), which provided the HEXE hard X-ray spectrometer, sensitive in the
20–120 keV energy band, (iii) the European Space Research and Technology
Center (ESTEC) of ESA, which built the GSPS high-pressure-gas spectrometer.
IKI constructed the Pulsar X-1 hard X-ray detector for this project.

The Kvant module was put into orbit by a Proton rocket and successfully docked
with the space station in April 1987. The main scientific result of the
Roentgen observatory was the discovery in August 1987 of hard X-ray emission
with an unusual spectrum from the very bright supernova SN 1987A, which had
exploded five months earlier in the Large Magellanic Cloud (Sunyaev et al.
1987; Syunyaev et al. 1987). The detected photons were emitted as gamma-ray
lines associated with the decay of radioactive cobalt 56. During the passage
through the optically thick shell of the supernova, these photons experienced
multiple Compton scatterings off relatively cold electrons and lost their
energy due to recoil. As a result, an extremely hard power-law X-ray spectrum
formed, and this spectrum was detected for several months by HEXE and Pulsar
X-1. At energies below 20 keV, photoabsorption of photons by ions of heavy
elements, first of all iron and cobalt, came into play. This explained the
absence of a detectable signal by TTM.

In 1987–1995, TTM discovered many transient and persistent sources, which now
wear the names “KS” (Kvant source, Sunyaev et al. 1991). The availability of
instruments sensitive in different bands of the X-ray spectrum allowed
broadband spectra in the 2–200 keV energy range to be constructed for very
many bright transient and persistent sources in binary stellar systems,
including black holes and neutron stars (X-ray pulsars and neutron stars with
weak magnetic fields) (Syunyaev et al. 1991, 1994). It was demonstrated for
the first time how strongly the spectra of X-ray binaries depend on the nature
of the accreting object. TTM provided excellent timing observations of X-ray
pulsars (Gilfanov et al. 1989) and high-quality X-ray images of the Galactic
center region.

The second X-ray orbital observatory of IKI within the Intercosmos program was
the GRANAT satellite, built by NPO Lavochkin and put into an elongated
elliptical orbit around Earth with a four-day period by a Proton rocket. It
operated in orbit from December 1989 to May 1999. The payload of GRANAT
included the SIGMA hard X-ray and gamma-ray telescope with a germanium
spectrometer, the PHEBUS gamma-ray burst detectors developed in France, the
WATCH all-sky monitor (Denmark), and the ART-P instrument constructed by IKI
and its subsidiary in Frunze (now Bishkek, Kyrgyzstan), sensitive in the 2–25
keV energy band. Both SIGMA and the ART-P telescopes had a coded mask and
position-sensitive detectors, permitting construction of the images.

The main results of the GRANAT mission include the maps of the central region
of the galaxy in hard X-rays by the SIGMA (40–100 keV) and ART-P (3–30 keV)
telescopes (e.g., Churazov et al. 1994; Gilfanov et al. 1993; Pavlinsky et al.
1994). WATCH discovered the extremely interesting black hole X-ray binary GRS
1915$-$105 (“GRS” means that it is a GRANAT source, Castro-Tirado et al.
1992). The spatial correlation of the 8–22 keV X-ray emission observed by
ART-P with the distribution of molecular gas in the Galactic center region
became the first indication of the X-ray echo of the past Sgr A* X-ray
activity (Sunyaev et al. 1993) that is scattered by H2 molecules in the dense
gas surrounding it, according to the prediction by Vainshtein & Syunyaev
(1980). PHEBUS observations helped to clearly demonstrate (Tkachenko et al.
2002) the existence of two types of gamma-ray bursts: short and normal, which,
as we know now, is closely related to the parameters of the stellar objects
that are responsible for their appearance (merger of neutron stars in the case
of short bursts and collapse of massive stars in the case of normal bursts).

In 2002, according to the agreement between ESA and Roscosmos, a Proton rocket
placed the International Gamma-ray Laboratory (INTEGRAL) into a high-apogee
orbit. INTEGRAL continues to provide scientific data to this day. Russian
scientists are granted 25% of the observing time of this mission. The
prominent results of INTEGRAL include the detailed spectroscopy of the
electron-positron annihilation radiation (Churazov et al. 2005) and the first
detection of gamma-ray lines from the type Ia supernova SN2014J (Churazov et
al. 2014).

###  8.2 Beginning of the SRG project in the USSR

In 1987, the thirtieth anniversary of the launch of the first artificial
satellite of Earth (Sputnik) was celebrated. On this occasion, a large
international meeting was held at IKI, where Intercosmos announced a
competition for the payload of two orbital observatories. This resulted in a
decision to support projects Radioastron and Spectrum-Roentgen-Gamma (SRG).
The SRG project was supported by a number of renowned Soviet physicists,
including Ya.B. Zeldovich, A.D. Sakharov, and the IKI director R.Z. Sagdeev.
In addition to the High Energy Astrophysics Department of IKI, which proposed
the SRG project, it was joined by Denmark, the UK, Italy, MPE (Germany), NASA
(USA), Finland, Switzerland, Israel, and Turkey. The work on the project was
in full swing (see Fig. 45), but huge changes in the USSR led to a slowdown in
the works on the spacecraft and instruments in 2001–2002, after which the
project was terminated.

Figure 45: Former SRG project.

In 2003, the Space Council of the Russian Academy of Sciences considered and
supported a more modest project with a launch by a Soyuz rocket (instead of a
Proton), a smaller number of instruments, and with a shift in the scientific
goals toward cosmology, fine X-ray spectroscopy, and monitoring the whole sky
in X-rays. The idea of attracting international cooperation to the project was
also supported. The project was then considered by Roscosmos, and a key
decision was made to elaborate it further. It was also decided to retain (for
the sake of continuity) the former project’s name SRG, although the new
project did not foresee the presence of instruments sensitive in the gamma-ray
range.

Among the first invited to discuss a possible participation in the project
were scientists from the MPE in Germany, with whom the High Energy
Astrophysics Department of IKI has closely worked during the Roentgen mission
on Mir/Kvant and on the first version of SRG. Moreover, MPE had developed and
built the X-ray telescope for the extremely successful ROSAT satellite, which
obtained wonderful X-ray maps of the sky in 1990.

MPE proposed using a modification of the telescope that was developed by MPE
for the unfortunately failed ABRIXAS mission for observations of clusters of
galaxies (the most massive objects in the Universe, which are of interest to
the Russian side as well). This telescope and its detector were designed for
exploring the sky in a significantly harder energy band (from 0.5 to 10 keV)
compared to ROSAT (see a more detailed discussion in §7.1). By this time, the
number of massive clusters of galaxies required for the detection of baryonic
acoustic oscillations in their spatial distribution had been calculated at the
Max Planck Institute for Astrophysics (MPA) and was proved to be close to
100,000 (Hütsi 2006). This would require finding virtually all massive
clusters of galaxies in the observable Universe. This question was actively
discussed at MPA in connection with searches for the baryonic acoustic
oscillations predicted by theorists (Peebles & Yu 1970; Sunyaev & Zeldovich
1970) and with emerging hopes to find a large number of galaxy clusters
through the Sunyaev–Zeldovich effect using ground-based telescopes and
spacecraft operating in the microwave band. Therefore the question arose about
a telescope with a higher sensitivity and working in a softer X-ray energy
band compared to the existing modifications of the instrument for the ABRIXAS
satellite, which could not provide the necessary sensitivity to fulfill this
task, nor the majority of other tasks that were of interest for cosmologists
at the time.

After long discussions, the Russian side agreed on drastic increases in the
size, mass, (800 kg) and power consumption of the German instrument to fulfil
the requirements (according to elaborations by G. Hasinger and P. Predehl)
posed by the planned scientific results of the sky survey. The parameters of
the Russian ART-XC instrument were also growing in parallel.

On March 23, 2007, Roscosmos and DLR signed a memorandum about the inclusion
of the eROSITA instrument into the payload of the SRG orbital observatory.
This opened the way to intensive work on the project and the telescope.

###  8.3 Data sharing between the participants of the SRG project

All data obtained by the ART-XC telescope belong to the scientists of IKI, who
have developed the telescope. The data within an area of 200 square degrees
around the north ecliptic pole (where the sensitivity of the all-sky survey is
highest) are processed together by scientists at IKI and the MSFC, in exchange
for the delivery by the latter of a subset of the ART-XC X-ray mirrors.

According to the memorandum of 2007 between Roscosmos and DLR, the SRG/eROSITA
data belong in equal parts to the German and Russian scientists. Because the
primary goal of SRG/eROSITA consists of constructing all-sky X-ray maps and
creating X-ray source catalogs, a decision has been made that the German
scientists are responsible for the processing and publication of data in one
hemisphere of the sky, and the Russian scientists provide this in the other
hemisphere (see Fig. 46).

Figure 46: Division of the SRG/eROSITA data between the German and Russian
consortia (denoted MPE and IKI, respectively). The border vertically follows
in Galactic coordinates through the central supermassive black hole Sgr A*.

##  9 Conclusion

The SRG observatory has been operating in orbit for almost two years at the
time of writing. The ART-XC and eROSITA telescopes have already scanned 40% of
the entire sky for the fourth time. The experience of 25 months of operation
of SRG allows us to say that the initial plans of the observatory are being
successfully implemented and the number of extragalactic objects being
discovered corresponds to the expectations before the launch. Using the new
data obtained in the fourth scan of the sky, it is possible to search for
sources whose X-ray brightness has strongly changed over the period of 6–18
months that passed since their previous appearances in the telescopes’ FoV
during the first three all-sky surveys. Every day, the observatory discovers
about five to ten objects within a $360^{\circ}\times 1^{\circ}$ strip (i.e.,
less than 1%) of the sky whose brightness has increased more than ten times
over 6 months. The depth of the X-ray maps and the number of detected sources
continues to increase with increasing exposure. The observatory is continuing
its round-the-clock mission.

##  10 Acknowledgements

This work is based on observations with the eROSITA and ART-XC telescopes
aboard the SRG observatory. The SRG observatory was built by Roskosmos in the
interests of the Russian Academy of Sciences represented by its Space Research
Institute (IKI) in the framework of the Russian Federal Space Program, with
the participation of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The
SRG/eROSITA X-ray telescope was built by a consortium of German Institutes led
by MPE, and supported by DLR. The SRG spacecraft was designed, built,
launched, and is operated by the Lavochkin Association and its subcontractors.
The science data are downlinked via the Deep Space Network antennas in Bear
Lakes, and Ussurijsk, funded by Roskosmos. The eROSITA data used in this work
were processed using the eSASS software system developed by the German eROSITA
consortium and proprietary data reduction and analysis software developed by
the Russian eROSITA Consortium.

## References

  * Ackermann et al. (2014) Ackermann, M., Albert, A., Atwood, W. B., et al. 2014, ApJ, 793, 64