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We present an enhanced version of the publicly available Stripe 82X catalog (S82-XL), featuring a comprehensive set of 22,737 unique X-ray point sources identified with a significance ≳4 σ. This catalog is four times larger than the original Stripe 82X catalog, by including additional archival data from the Chandra and XMM-Newton telescopes. Now covering ∼54.8 deg2 of nonoverlapping sky area, the S82-XL catalog roughly doubles the area and depth of the original catalog, with limiting fluxes (half-area fluxes) of 3.4 × 10−16 (2.4 × 10−15), 2.9 × 10−15 (1.5 × 10−14), and 1.4 × 10−15 (9.5 × 10−15) erg s−1 cm−2 across the soft (0.5–2 keV), hard (2–10 keV), and full (0.5–10 keV) bands, respectively. S82-XL occupies a unique region of flux-area parameter space compared to other X-ray surveys, identifying sources with rest-frame luminosities from 1.2 × 1038 to 1.6 × 1047 erg s−1 in the 2–10 keV band (median X-ray luminosity, 7.2 × 1043 erg s−1), and spectroscopic redshifts up to z ∼ 6. By using hardness ratios, we derived the obscuration of active galactic nuclei (AGNs), obtaining a median value of log (N H / cm − 2) = 21.6 − 1.6 + 1.0 and an overall, obscured fraction ( log (N H / cm − 2) > 22 ) of ∼36.9%. S82-XL serves as a benchmark in X-ray surveys and, with its extensive multiwavelength data, is especially valuable for comprehensive studies of luminous AGNs. [ABSTRACT FROM AUTHOR]

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within 10 3 − 6 M ⊙ , are commonly found at the centers of dwarf galaxies and may exist, yet undiscovered, in the MW. By using model spectra for advection-dominated accretion flows (ADAFs), we calculated the expected fluxes emitted by a population of wandering IMBHs with masses of 10 5 M ⊙ in various MW environments and extrapolated our results to massive local galaxies. Around 40 % of the potential population of wandering IMBHs in the MW can be detected in an AXIS deep field. We proposed criteria to aid with selecting IMBH candidates using already available optical surveys. We also showed that IMBHs wandering in >200 galaxies within 10 Mpc can be easily detected with AXIS when passing within dense galactic environments (e.g., molecular clouds and cold neutral medium). In summary, we highlighted the potential X-ray detectability of wandering IMBHs in local galaxies and provided insights for guiding future surveys. Detecting wandering IMBHs is crucial for understanding their demographics and evolution and the merging history of galaxies. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website. [ABSTRACT FROM AUTHOR]

Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high-density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early universe by unveiling the first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs in the past ∼50 years have stimulated the development of cutting-edge technological instruments for observations of high-energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data-taking mode currently). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and the associated phenomenology. [ABSTRACT FROM AUTHOR]

There are two contradictory views of the eROSITA bubbles: either a 10 ^4 pc scale pair of giant bubbles blown by the Galactic center (GC), or a 10 ^2 pc scale local structure coincidentally located in the direction of GC. A key element of this controversy is the distance to the bubbles. Based on the 3D dust distribution in the Galactic plane, we found three isolated, distant (500–800 pc) clouds at intermediate Galactic latitudes. Their projected morphologies perfectly match the X-ray shadows on the defining features of the north eROSITA bubble, i.e., the North Polar Spur (NPS) and the Lotus Petal Cloud (LPC), indicating that both the NPS and LPC are distant, with a distance lower limit of nearly 1 kpc. In the X-ray-dark region between the NPS and LPC, we found a few polarized radio arcs and attributed them to the bubble’s shock front. These arcs match up perfectly with the outer border of the NPS and LPC and provide a way to define the bubble’s border. The border defined in this way can be well described by the line-of-sight tangent of a 3D skewed cup model rooted in the GC. We conclude that, instead of being two independent, distant features, the NPS and LPC compose a single, giant bubble, which therefore is most plausibly a 10 kpc scale bubble rooted at the GC.

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within 103−6M⊙, are commonly found at the centers of dwarf galaxies and may exist, yet undiscovered, in the MW. By using model spectra for advection-dominated accretion flows (ADAFs), we calculated the expected fluxes emitted by a population of wandering IMBHs with masses of 105M⊙ in various MW environments and extrapolated our results to massive local galaxies. Around 40% of the potential population of wandering IMBHs in the MW can be detected in an AXIS deep field. We proposed criteria to aid with selecting IMBH candidates using already available optical surveys. We also showed that IMBHs wandering in >200 galaxies within 10 Mpc can be easily detected with AXIS when passing within dense galactic environments (e.g., molecular clouds and cold neutral medium). In summary, we highlighted the potential X-ray detectability of wandering IMBHs in local galaxies and provided insights for guiding future surveys. Detecting wandering IMBHs is crucial for understanding their demographics and evolution and the merging history of galaxies. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website.

Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high-density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early universe by unveiling the first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs in the past ∼50 years have stimulated the development of cutting-edge technological instruments for observations of high-energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data-taking mode currently). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and the associated phenomenology.

Modern X-ray telescopes have detected hundreds of thousands of X-ray sources in the universe. However, current methods to classify these sources using the X-ray data themselves suffer problems—detailed X-ray spectroscopy of individual sources is too time consuming, while hardness ratios often lack accuracy, and can be difficult to use effectively. These methods fail to use the power of X-ray CCD detectors to identify X-ray emission lines and distinguish line-dominated spectra (from chromospherically active stars, supernova remnants, etc.) from continuum-dominated ones (e.g., compact objects or active galactic nuclei, AGN). In this paper, we probe the use of artificial neural networks (ANN) in differentiating Chandra spectra of young stars in the Chandra Orion Ultradeep Project (COUP) survey from AGN in the Chandra Deep Field South (CDFS) survey. We use these surveys to generate 100,000 artificial spectra of stars and AGN, and train our ANN models to separate the two kinds of spectra. We find that our methods reach an accuracy of ∼92% in classifying simulated spectra of moderate-brightness objects in typical exposures, but their performance decreases on the observed COUP and CDFS spectra (∼91%), due in large part to the relatively high background of these long-exposure data sets. We also investigate the performance of our methods with changing properties of the spectra such as the net source counts, the relative contribution of background, the absorption column of the sources, etc. We conclude that these methods have substantial promise for application to large X-ray surveys. [ABSTRACT FROM AUTHOR]

We explore the evolution of the ∼10 ^7 K hot gas in normal galaxies out to redshift = 0.5 (lookback time = 5 Gyr), using X-ray luminosity functions (XLFs) built from a sample of 575 normal galaxies with z < 0.6 detected in five high-galactic-latitude Chandra wide-field surveys. After estimating the emission due to the hot gas component (reducing the sample to ∼400 galaxies), we compared the XLF in three redshift bins ( z = 0.1, 0.3, and 0.5), finding increases in the number of galaxies per unit comoving volume from z = 0.1 to 0.3 and then from z = 0.3 to 0.5. These XLF changes suggest a significant (∼5 σ ) X-ray luminosity evolution of the hot gas, with L _X,GAS decreasing by a factor of 6–10 in the last 5 Gyr (from z = 0.5 to 0.1). The relative abundance of L _X,GAS ∼ 10 ^41 erg s ^−1 galaxies at higher z suggests that high -z , moderate -L _X,GAS galaxies may be the optimal target to solve the missing baryon problem. In early-type galaxies, this observational trend is qualitatively consistent with (but larger than) the expected time-dependent mass-loss rate in cooling flow models without active galactic nucleus feedback. In late-type galaxies, the observational trend is also qualitatively consistent with (but larger than) the effect of the z -dependent star formation rate.

We present an enhanced version of the publicly available Stripe 82X catalog (S82-XL), featuring a comprehensive set of 22,737 unique X-ray point sources identified with a significance ≳4 σ . This catalog is four times larger than the original Stripe 82X catalog, by including additional archival data from the Chandra and XMM-Newton telescopes. Now covering ∼54.8 deg ^2 of nonoverlapping sky area, the S82-XL catalog roughly doubles the area and depth of the original catalog, with limiting fluxes (half-area fluxes) of 3.4 × 10 ^−16 (2.4 × 10 ^−15 ), 2.9 × 10 ^−15 (1.5 × 10 ^−14 ), and 1.4 × 10 ^−15 (9.5 × 10 ^−15 ) erg s ^−1 cm ^−2 across the soft (0.5–2 keV), hard (2–10 keV), and full (0.5–10 keV) bands, respectively. S82-XL occupies a unique region of flux-area parameter space compared to other X-ray surveys, identifying sources with rest-frame luminosities from 1.2 × 10 ^38 to 1.6 × 10 ^47 erg s ^−1 in the 2–10 keV band (median X-ray luminosity, 7.2 × 10 ^43 erg s ^−1 ), and spectroscopic redshifts up to z ∼ 6. By using hardness ratios, we derived the obscuration of active galactic nuclei (AGNs), obtaining a median value of ${\rm{log}}({N}_{{\rm{H}}}/{{\rm{cm}}}^{-2})={21.6}_{-1.6}^{+1.0}$ and an overall, obscured fraction ( ${\rm{log}}({N}_{{\rm{H}}}/{{\rm{cm}}}^{-2})\gt 22$ ) of ∼36.9%. S82-XL serves as a benchmark in X-ray surveys and, with its extensive multiwavelength data, is especially valuable for comprehensive studies of luminous AGNs.

We present a catalog of hard X-ray serendipitous sources detected in the first 80 months of observations by the Nuclear Spectroscopic Telescope Array (NuSTAR). The NuSTAR serendipitous survey 80 month (NSS80) catalog has an unprecedented ∼62 Ms of effective exposure time over 894 unique fields (a factor of 3 increase over the 40 month catalog, NSS40), with an areal coverage of ∼36 deg ^2 , larger than all NuSTAR extragalactic surveys. NSS80 provides 1274 hard X-ray sources in the 3−24 keV band (822 new detections compared to the previous NSS40). Approximately 76% of the NuSTAR sources have lower-energy (

We present the second NuSTAR and XMM-Newton extragalactic survey of the JWST north ecliptic pole (NEP) time-domain field (TDF). The first NuSTAR NEP-TDF survey had 681 ks total exposure time executed in NuSTAR cycle 5 in 2019 and 2020. This second survey, acquired from 2020 to 2022 in cycle 6, adds 880 ks of NuSTAR exposure time. The overall NuSTAR NEP-TDF survey is the most sensitive NuSTAR extragalactic survey to date, and a total of 60 sources were detected above the 95% reliability threshold. We constrain the hard X-ray number counts, $\mathrm{log}N$ – $\mathrm{log}S$ , down to 1.7 × 10 ^−14 erg cm ^−2 s ^−1 at 8–24 keV and detect an excess of hard X-ray sources at the faint end. About 47% of the NuSTAR-detected sources are heavily obscured ( N _H > 10 ^23 cm ^−2 ), and ${18}_{-8}^{+20}$ % of the NuSTAR-detected sources are Compton-thick ( N _H > 10 ^24 cm ^−2 ). These fractions are consistent with those measured in other NuSTAR surveys. Four sources presented >2 σ variability in the 3 yr survey. In addition to NuSTAR, a total of 62 ks of XMM-Newton observations were taken during NuSTAR cycle 6. The XMM-Newton observations provide soft X-ray (0.5–10 keV) coverage in the same field and enable more robust identification of the visible and infrared counterparts of the NuSTAR-detected sources. A total of 286 soft X-ray sources were detected, out of which 214 XMM-Newton sources have secure counterparts from multiwavelength catalogs.

In the latest data release from the Fermi Gamma-ray Space Telescope (the 4th Fermi LAT 14 yr Catalog, or 4FGL), more than 50% of the Galactic sources are yet to be identified. We observed 13 unidentified Fermi LAT sources with the Chandra X-Ray Observatory to explore their nature. We report the results of the classification of X-ray sources in the fields of these γ -ray sources and discuss the implications for their nature. We use multiwavelength (MW) data for a machine-learning classification, accompanied by a more detailed spectral/variability analysis for brighter sources. Eight 4FGL sources have γ -ray pulsars within their position error ellipses. We consider three of these pulsars (PSR J1906+0722, PSR J1105–6037, and PSR J1358–6025) to be detected in X-rays, while PSR J1203–6242 shows a hint of X-ray emission. Within the positional uncertainties of three of the 4FGL sources, we detect X-ray sources that may be yet unknown pulsars, depending on the MW association. In addition to point sources, we discovered two extended sources, one of which is likely to be a bow-shock pulsar-wind nebula associated with PSR J1358–6025. Finally, we classify other X-ray sources detected in these observations and report the most interesting classifications.

The current sample of 12 radio-quiet isolated neutron stars that emit strongly in X-rays (XINSs) is both small and heterogeneous, limiting its usefulness for understanding the physics of neutron star atmospheres and cooling rates and for constraining the equation of state of neutron degenerate matter. Utilizing the ROSAT 1RXS and 2RXS data sets, in conjunction with the Sloan Digital Sky Survey Data Release 17 and other companion multiwavelength surveys, we have extended previous searches for blank-field X-ray source candidate XINSs, ultimately recovering two known XINSs while identifying 46 new, unstudied candidate fields devoid of likely multiwavelength counterparts. In this publication, we describe our selection approach and provide detailed information regarding our sample of new candidate XINSs. Future opportunities to verify or to refute these X-ray sources as isolated neutron stars by obtaining more accurate X-ray source positions, quality X-ray spectra, or deeper optical imaging are also discussed.

The four completed six-month sky surveys with the eROSITA telescope onboard the SRG orbital observatory allow tidal disruption events (TDEs) in galactic nuclei to be searched for by their X-ray variability. In this case, variable active galactic nuclei (AGNs) are detected in much larger quantities, for the elimination of which fairly stringent criteria have to be used. Some TDEs can be missed or misclassified as probable AGNs. Optical spectroscopy needs to be performed for the final identification of TDEs among the extragalactic SRG/eROSITA transients. We consider a set of criteria by which TDEs can be distinguished from AGNs based on optical spectra and photometric information. In particular, we propose to use the ratio of the X-ray luminosity to the [O III] 5007 Å luminosity. To test the proposed method, we consider a sample of 15 extragalactic SRG/eROSITA transients whose X-ray flux changed by more than a factor of 7 between two adjacent surveys. Spectra have been taken for all these objects with Russian optical telescopes: RTT-150, AZT-33IK, RC2500, and BTA. We have managed to reveal five new and one previously known TDEs and to classify seven sources as AGNs. The nature of two more transients remains in question. The proposed method will help to set priorities to obtain a maximally complete and reliable sample of TDEs in the SRG/eROSITA survey. [ABSTRACT FROM AUTHOR]

We discuss the results of our spectroscopy for 17 candidates for distant quasars selected by the SRGz classification system based on data from the first and second sky surveys with the eROSITA telescope onboard the SRG X-ray observatory. Most of the quasar candidates were selected within the DaLeQo observing program, whose goal is a search for the most X-ray luminous quasars with luminosities erg s at redshifts . Our spectroscopic observations were carried out at the 6-m BTA telescope with the SCORPIO-2 spectrograph. We have managed to confirm eight new X-ray quasars at , including the quasar SRGe J020142.8-015347 at . Owing to the large diameter of the BTA mirror and the high quantum efficiency of the SCORPIO-2 spectrograph in the range 6000–9000 Å, we have managed to measure the С IV (1549 Å) line parameters and to estimate the supermassive black hole masses and accretion rates for seven quasars. [ABSTRACT FROM AUTHOR]

The Low-Energy X-ray Polarization Detector (LPD) is one of the payloads in the POLAR-2 experiment, designed as an external payload for the China Space Station deployment in early 2026. LPD is specifically designed to observe the polarization of gamma-ray burst (GRB) prompt emission in the energy range of 2–10 keV, with a wide field of view (FoV) of 90° in preliminary design. This observation is achieved using an array of X-ray photoelectric polarimeters based on gas pixel detectors. Due to the wide FoV configuration, the in-orbit background count rate in the soft X-ray range is high, while GRBs themselves also exhibit high flux in this energy band. In order to assess the contribution of various background components to the total count rate, we conducted detailed simulations using the GEANT4 C++ package. Our simulations encompassed the main interactions within the instrument materials and provided insights into various background components within the wide-FoV scheme. The simulation results reveal that among the background components, the primary contributors are the cosmic X-ray background (CXB) and bright X-ray sources. The total background count rate of LPD, after applying the charged particle background rejection algorithm, is approximately 0.55 counts cm ^–2 s ^–1 on average, and it varies with the detector’s orbit and pointing direction. Furthermore, we performed comprehensive simulations and comparative analyses of the CXB and X-ray bright sources under different FoVs and detector pointings. These analyses provide valuable insights into the background characteristics for soft X-ray polarimeter with wide FoV.

We present two independent measurements of stellar velocity dispersions ( σ _⋆ ) from the Ca ii H+K λ 3969, 3934 and Mg i b λ 5183, 5172, 5167 region (3880–5550 Å) and the calcium triplet region (8350–8750 Å) for 173 hard X-ray-selected Type 1 active galactic nuclei (AGNs; z ≤ 0.08) from the 105 month Swift-BAT catalog. We construct one of the largest samples of local Type 1 AGNs that have both single-epoch virial black hole mass ( M _BH ) estimates and σ _⋆ measurements obtained from high spectral resolution data, allowing us to test the usage of such methods for supermassive black hole studies. We find that the two independent σ _⋆ measurements are highly consistent with each other, with an average offset of only 0.002 ± 0.001 dex. Comparing M _BH estimates based on broad emission lines and stellar velocity dispersion measurements, we find that the former is systematically lower by ≈0.12 dex. Consequently, Eddington ratios estimated through broad-line M _BH determinations are similarly biased (but in the opposite way). We argue that the discrepancy is driven by extinction in the broad-line region. We also find an anticorrelation between the offset from the M _BH – σ _⋆ relation and the Eddington ratio. Our sample of Type 1 AGNs shows a shallower M _BH – σ _⋆ relation (with a power-law exponent of ≈3.5) compared with that of inactive galaxies (with a power-law exponent of ≈4.5), confirming earlier results obtained from smaller samples.

X-ray bright optically normal galaxies (XBONGs) are galaxies with X-ray luminosities consistent with those of active galactic nuclei (AGNs) but no evidence of AGN optical emission lines. Crossmatching the Chandra Source Catalog version 2 with the Sloan Digital Sky Survey sample of spectroscopically classified galaxies, we have identified 817 XBONG candidates with L _X > 10 ^42 erg s ^−1 and X-ray to optical flux ratio F _XO > 0.1. Comparisons with Wide-field Infrared Survey Explorer colors and near-IR, optical, UV, and radio luminosities show that the loci of XBONGs are in-between those of control samples of normal galaxies and quasars and are consistent with low-luminosity quasars. We find that 43% of the XBONG sample have X-ray colors suggesting N _H > 10 ^22 cm ^−2 , double the fraction in the QSO sample, suggesting that a large fraction of XBONG are highly obscured AGNs. However, ∼50% of the XBONGs are not obscured and have X-ray colors harder than those of normal galaxies. Some of these XBONGs have spatially extended X-ray emission. These characteristics suggest that they may be unidentified galaxy groups and clusters. Using the X-ray luminosity functions of QSOs, galaxies, groups and clusters, we estimate the approximate fraction of extended XBONGs to be

AM Canum Venaticorum (AM CVn) systems are ultracompact binaries where a white dwarf accretes from a helium-rich degenerate or semidegenerate donor. Some AM CVn systems will be among the loudest sources of gravitational waves for the upcoming Laser Interferometer Space Antenna; yet the formation channel of AM CVns remains uncertain. We report the study and characterization of a new eclipsing AM CVn, SRGeJ045359.9+622444 (hereafter, SRGeJ0453), discovered from a joint Spektrum-Roentgen-Gamma (SRG) Extended Roentgen Survey with an Imaging Telescope Array (eROSITA) mission and Zwicky Transient Facility program to identify cataclysmic variables (CVs). We obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine the orbital period to be ${P}_{\mathrm{orb}}=55.0802\pm 0.0003\,\min $ . We constrain the binary parameters by modeling the high-speed photometry and radial-velocity curves and find M _donor = 0.044 ± 0.024 M _⊙ and R _donor = 0.078 ± 0.012 R _⊙ . The X-ray spectrum is approximated by a power-law model with an unusually flat photon index of Γ ∼ 1 previously seen in magnetic CVs with SRG/eROSITA, but verifying that the magnetic nature of SRGeJ0453 requires further investigation. Optical spectroscopy suggests that the donor star of SRGeJ0453 could have initially been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn system published to date, and its lack of optical outbursts have made it elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and optical surveys highlights the potential for discovering similar systems in the near future.

By characterizing the contribution of stray light to large data sets from the CXB Measurement X-ray observatory collected over 2012–2017, we report a measurement of the cosmic X-ray background (CXB) in the 3–20 keV energy range. These data represent ∼20% sky coverage while avoiding Galactic ridge X-ray emission and are less weighted by deep survey fields than previous measurements with CXB Measurement. Images in narrow energy bands are stacked in detector space and spatially fit with a model representing the stray light and uniform pattern expected from the CXB and the instrumental background, respectively. We establish baseline flux values from Earth-occulted data and validate the fitting method on stray-light observations of the Crab, which further serve to calibrate the resulting spectra. We present independent spectra of the CXB with the focal plane module FPMA and FPMB detector arrays, which are in excellent agreement with the canonical characterization by HEAO 1 and are 10% lower than most subsequent measurements: ${F}_{3-20\,\mathrm{keV}}^{\mathrm{FPMA}}=2.63\times {10}^{-11}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}\,{\deg }^{-2}$ and ${F}_{3\mbox{--}20\,\mathrm{keV}}^{\mathrm{FPMB}}\,=2.58\times {10}^{-11}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}\,{\deg }^{-2}$ . We discuss these results in light of previous measurements of the CXB and consider the impact of systematic uncertainties on our spectra.

We present new estimates on the fraction of heavily X-ray-obscured, Compton-thick (CT) active galactic nuclei (AGNs) out to a redshift of z ≤ 0.8. From a sample of 540 AGNs selected by mid-infrared (MIR) properties in observed X-ray survey fields, we forward model the observed-to-intrinsic X-ray luminosity ratio ( ${R}_{{L}_{{\rm{X}}}}$ ) with a Markov Chain Monte Carlo simulation to estimate the total fraction of CT AGNs ( f _CT ), many of which are missed in typical X-ray observations. We create model N _H distributions and convert these to ${R}_{{L}_{{\rm{X}}}}$ using a set of X-ray spectral models. We probe the posterior distribution of our models to infer the population of X-ray-nondetected sources. From our simulation we estimate a CT fraction of f _CT = ${0.555}_{-0.032}^{+0.037}$ . We perform an X-ray stacking analysis for sources in Chandra X-ray Observatory fields and find that the expected soft (0.5–2 keV) and hard (2–7 keV) observed fluxes drawn from our model to be within 0.48 and 0.12 dex of our stacked fluxes, respectively. Our results suggests at least 50% of all MIR-selected AGNs, possibly more, are CT ( N _H ≳ 10 ^24 cm ^−2 ), which is in excellent agreement with other recent work using independent methods. This work indicates that the total number of AGNs is higher than can be identified using X-ray observations alone, highlighting the importance of a multiwavelength approach. A high f _CT also has implications for black hole (BH) accretion physics and supports models of BH and galaxy coevolution that include periods of heavy obscuration.

Modern X-ray telescopes have detected hundreds of thousands of X-ray sources in the universe. However, current methods to classify these sources using the X-ray data themselves suffer problems—detailed X-ray spectroscopy of individual sources is too time consuming, while hardness ratios often lack accuracy, and can be difficult to use effectively. These methods fail to use the power of X-ray CCD detectors to identify X-ray emission lines and distinguish line-dominated spectra (from chromospherically active stars, supernova remnants, etc.) from continuum-dominated ones (e.g., compact objects or active galactic nuclei, AGN). In this paper, we probe the use of artificial neural networks (ANN) in differentiating Chandra spectra of young stars in the Chandra Orion Ultradeep Project (COUP) survey from AGN in the Chandra Deep Field South (CDFS) survey. We use these surveys to generate 100,000 artificial spectra of stars and AGN, and train our ANN models to separate the two kinds of spectra. We find that our methods reach an accuracy of ∼92% in classifying simulated spectra of moderate-brightness objects in typical exposures, but their performance decreases on the observed COUP and CDFS spectra (∼91%), due in large part to the relatively high background of these long-exposure data sets. We also investigate the performance of our methods with changing properties of the spectra such as the net source counts, the relative contribution of background, the absorption column of the sources, etc. We conclude that these methods have substantial promise for application to large X-ray surveys.

We present X-ray spectral analysis of XMM-Newton and Chandra observations in the 31.3 deg ^2 Stripe-82X (S82X) field. Of the 6181 unique X-ray sources in this field, we analyze a sample of 2937 candidate active galactic nuclei (AGNs) with solid redshifts and sufficient counts determined by simulations. Our results show an observed population with median values of spectral index ${\rm{\Gamma }}={1.94}_{-0.39}^{+0.31}$ , column density log $\,{N}_{{\rm{H}}}/{\mathrm{cm}}^{-2}={20.7}_{-0.5}^{+1.2}$ and intrinsic, de-absorbed, 2–10 keV luminosity log $\,{L}_{{\rm{X}}}/\mathrm{erg}\,\ {{\rm{s}}}^{-1}={44.0}_{-1.0}^{+0.7}$ , in the redshift range 0–4. We derive the intrinsic, model-independent, fraction of AGNs that are obscured ( $22\leqslant \mathrm{log}\,{N}_{{\rm{H}}}/{\mathrm{cm}}^{-2}\lt 24$ ), finding a significant increase in the obscured AGN fraction with redshift and a decline with increasing luminosity. The average obscured AGN fraction is 57% ± 4% for log L _X /erg s ^−1 > 43. This work constrains the AGN obscuration and spectral shape of the still uncertain high-luminosity and high-redshift regimes (log L _X /erg s ^−1 > 45.5, z > 3), where the obscured AGN fraction rises to 64% ± 12%. We report a luminosity and density evolution of the X-ray luminosity function, with obscured AGNs dominating at all luminosities at z > 2, and unobscured sources prevailing at log L _X /erg s ^−1 > 45 at lower redshifts. Our results agree with the evolutionary models in which the bulk of AGN activity is triggered by gas-rich environments and in a downsizing scenario. Moreover, the black hole accretion density (BHAD) is found to evolve similarly to the star formation rate density, confirming the coevolution between AGN and host galaxy, but suggesting different timescales in their growing history. The derived BHAD evolution shows that Compton-thick AGNs contribute to the accretion history of AGNs as much as all other AGN populations combined.

Lobster eye telescopes are ideal monitors to detect X-ray transients because they could observe celestial objects over a wide field of view in the X-ray band. However, images obtained by lobster eye telescopes are modified by their unique point-spread functions, making it hard to design a high-efficiency target detection algorithm. In this paper, we integrate several machine-learning algorithms to build a target detection framework for data obtained by lobster eye telescopes. Our framework would first generate two 2D images with different pixel scales according to positions of photons on the detector. Then, an algorithm based on morphological operations and two neural networks would be used to detect candidates of celestial objects with different flux from these 2D images. Finally, a random forest algorithm will be used to pick up final detection results from candidates obtained by previous steps. Tested with simulated data of the Wide-field X-ray Telescope on board the Einstein Probe, our detection framework could achieve over 94% purity and over 90% completeness for targets with flux more than 3 mcrab (9.6 × 10 ^−11 erg cm ^−2 s ^−1 ) and more than 94% purity and moderate completeness for targets with lower flux at acceptable time cost. The framework proposed in this paper could be used as references for data processing methods developed for other lobster eye X-ray telescopes.

The first X-ray source catalog of the Insight-HXMT Galactic Plane (∣ b ∣ < 10°) Scanning Survey (GPSS) is presented based on the data accumulated from 2017 June to 2021 August. The 4 yr limit sensitivities at main energy bands can reach 8.2 × 10 ^−12 erg s ^−1 cm ^2 (2−6 keV), 4.21 × 10 ^−11 erg s ^−1 cm ^2 (7−40 keV), and 2.78 × 10 ^−11 erg s ^−1 cm ^2 (25−100 keV). More than 1300 sources have been monitored at a wide band (1−100 keV), of which 223 sources have a signal-to-noise ratio greater than 5. We combined the GPSS data of Insight-HXMT and MAXI and found it feasible to obtain more complete long-term light curves from their scanning results. The flux variabilities at different energy bands of the 223 bright sources are analyzed based on the excess variances. It is found that the fluxes of X-ray binaries are more active than those of supernova remnants and isolated pulsars. Different types of binaries, e.g., low-mass X-ray binaries (LMXBs), high-mass X-ray binaries (HMXBs), neutron star binaries, and black hole binaries, also distinctively show different regularities. In addition, the relations between the hardness ratio (HR) and excess variances, and HR and source types are analyzed. It is obvious that the HRs of HMXBs tend to be harder than those of LMXBs and HMXBs tend to be more active than those of LMXBs.

Dwarf galaxies are promising test beds for constraining models of supermassive and intermediate-mass black holes (MBHs) via their BH occupation fraction (BHOF). Disentangling seeding from the confounding effects of mass assembly over a Hubble time is a challenging problem that we tackle in this study with a suite of semianalytical models (SAMs). We show how the measured BHOF depends on the lowest BH mass or active galactic nucleus (AGN) luminosity achieved by a survey. To tell seeding models apart, we need to detect or model all AGNs brighter than 10 ^37 erg s ^−1 in galaxies of M _* ∼ 10 ^8−10 M _⊙ . Shallower surveys, like eRASS, cannot distinguish between seed models even with the compensation of a much larger survey volume. We show that the AMUSE survey, with its inference of the MBH population underlying the observed AGNs, strongly favors heavy seed models, growing with either a power-law Eddington ratio distribution function or one in which BH accretion is tied to the star formation rate (i.e., the AGN-main sequence, AGN-MS, model). These two growth channels can then be distinguished by the AGN luminosity function at >10 ^40 erg s ^−1 , with the AGN-MS model requiring more accretion than observed at z ∼ 0. Thus, current X-ray observations favor heavy seeds whose Eddington ratios follow a power-law distribution. The different models also predict different radio scaling relations, which we quantify using the fundamental plane of BH activity. We close with recommendations for the design of upcoming multiwavelength campaigns that can optimally detect MBHs in dwarf galaxies.

Based on observations with the transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the redshifts for 15 new quasars detected in X-rays by the eROSITA telescope of the SRG orbital observatory during the first three all-sky surveys. The redshifts of the sources lie in the range 0.5–4.156. We have estimated the masses of the central supermassive black holes and the bolometric luminosities for eight quasars from the width of the C IV 1549 Å emission line and the continuum flux at 1350 Å. For these objects we have also estimated the ratio of the X-ray luminosity to the Eddington limit to be a few percent. At the same time, the bolometric-to-Eddington luminosity ratio for them has turned out to be in a fairly narrow range, 30–40 . A large ratio points to an active growth of the mass of the central black holes in these objects. [ABSTRACT FROM AUTHOR]

During the first all-sky survey of the SRG orbital observatory, the X-ray source SRGE J170245.3+130104 was discovered with the eROSITA telescope on March 13–15, 2020. Its optical counterpart was identified by photometric attributes as a candidate for distant quasars at . The spectroscopic observations of the object carried out in August and September 2020 at the 6-m BTA telescope with the SCORPIO-II instrument confirmed that SRGE J170245.3+130104 is a quasar at redshift . According to the eROSITA data obtained during the first sky survey, the X-ray luminosity of the quasar is erg s in the 2–10 keV energy band, while its X-ray spectrum can be approximately fitted by a power law with a slope . The quasar was detected again with the eROSITA telescope half a year later (on September 13–14, 2020) during the second sky survey, with its X-ray luminosity, probably, having decreased approximately by a factor of 2 at a confidence level . SRGE J170245.3+130104 has turned out to be the most X-ray luminous quasar among all of the known quasars at redshifts . At the same time, it is also one of the radio loudest distant quasars (radio loudness ), which may imply that this is a blazar. We present a catalog of all the quasars at detected in X-rays to date. [ABSTRACT FROM AUTHOR]

We present the active galactic nucleus (AGN) catalog and optical spectroscopy for the second data release of the Swift BAT AGN Spectroscopic Survey (BASS DR2). With this DR2 release we provide 1449 optical spectra, of which 1182 are released for the first time, for the 858 hard-X-ray-selected AGNs in the Swift BAT 70-month sample. The majority of the spectra (801/1449, 55%) are newly obtained from Very Large Telescope (VLT)/X-shooter or Palomar/Doublespec. Many of the spectra have both higher resolution ( R > 2500, N ∼ 450) and/or very wide wavelength coverage (3200–10000 Å, N ∼ 600) that are important for a variety of AGN and host galaxy studies. We include newly revised AGN counterparts for the full sample and review important issues for population studies, with 47 AGN redshifts determined for the first time and 790 black hole mass and accretion rate estimates. This release is spectroscopically complete for all AGNs (100%, 858/858), with 99.8% having redshift measurements (857/858) and 96% completion in black hole mass estimates of unbeamed AGNs (722/752). This AGN sample represents a unique census of the brightest hard-X-ray-selected AGNs in the sky, spanning many orders of magnitude in Eddington ratio ( L / L _Edd = 10 ^−5 –100), black hole mass ( M _BH = 10 ^5 –10 ^10 M _⊙ ), and AGN bolometric luminosity ( L _bol = 10 ^40 –10 ^47 erg s ^−1 ).

We present new central stellar velocity dispersions for 484 Sy 1.9 and Sy 2 from the second data release of the Swift/BAT AGN Spectroscopic Survey (BASS DR2). This constitutes the largest study of velocity dispersion measurements in X-ray-selected obscured active galactic nuclei (AGN) with 956 independent measurements of the Ca ii H and K λ 3969, 3934 and Mg I λ 5175 region (3880–5550 Å) and the calcium triplet region (8350–8730 Å) from 642 spectra mainly from VLT/X-Shooter or Palomar/DoubleSpec. Our sample spans velocity dispersions of 40–360 km s ^1 , corresponding to 4–5 orders of magnitude in black hole mass ( M _BH = 10 ^5.5−9.6 M _⊙ ), bolometric luminosity ( L _bol ∼ 10 ^42–46 erg s ^−1 ), and Eddington ratio ( L / L _Edd ∼ 10 ^−5 to 2). For 281 AGN, our data and analysis provide the first published central velocity dispersions, including six AGN with low-mass black holes ( M _BH = 10 ^5.5−6.5 M _⊙ ), discovered thanks to high spectral resolution observations ( σ _inst ∼ 25 km s ^−1 ). The survey represents a significant advance with a nearly complete census of velocity dispersions of hard X-ray–selected obscured AGN with measurements for 99% of nearby AGN ( z < 0.1) outside the Galactic plane (∣ b ∣ > 10°). The BASS AGN have much higher velocity dispersions than the more numerous optically selected narrow-line AGN (i.e., ∼150 versus ∼100 km s ^−1 ) but are not biased toward the highest velocity dispersions of massive ellipticals (i.e., >250 km s ^−1 ). Despite sufficient spectral resolution to resolve the velocity dispersions associated with the bulges of small black holes (∼10 ^4–5 M _⊙ ), we do not find a significant population of super-Eddington AGN. Using estimates of the black hole sphere of influence from velocity dispersion, direct stellar and gas black hole mass measurements could be obtained with existing facilities for more than ∼100 BASS AGN.

We present measurements of broad emission lines and virial estimates of supermassive black hole masses ( M _BH ) for a large sample of ultrahard X-ray-selected active galactic nuclei (AGNs) as part of the second data release of the BAT AGN Spectroscopic Survey (BASS/DR2). Our catalog includes M _BH estimates for a total of 689 AGNs, determined from the H α , H β , Mg ii λ 2798, and/or C iv λ 1549 broad emission lines. The core sample includes a total of 512 AGNs drawn from the 70 month Swift/BAT all-sky catalog. We also provide measurements for 177 additional AGNs that are drawn from deeper Swift/BAT survey data. We study the links between M _BH estimates and line-of-sight obscuration measured from X-ray spectral analysis. We find that broad H α emission lines in obscured AGNs ( $\mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\gt 22.0$ ) are on average a factor of ${8.0}_{-2.4}^{+4.1}$ weaker relative to ultrahard X-ray emission and about ${35}_{-12}^{\,+7}$ % narrower than those in unobscured sources (i.e., $\mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\lt 21.5$ ). This indicates that the innermost part of the broad-line region is preferentially absorbed. Consequently, current single-epoch M _BH prescriptions result in severely underestimated (>1 dex) masses for Type 1.9 sources (AGNs with broad H α but no broad H β ) and/or sources with $\mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\gtrsim 22.0$ . We provide simple multiplicative corrections for the observed luminosity and width of the broad H α component ( L [bH α ] and FWHM[bH α ]) in such sources to account for this effect and to (partially) remedy M _BH estimates for Type 1.9 objects. As a key ingredient of BASS/DR2, our work provides the community with the data needed to further study powerful AGNs in the low-redshift universe.

The BAT AGN Spectroscopic Survey (BASS) is designed to provide a highly complete census of the key physical parameters of the supermassive black holes (SMBHs) that power local active galactic nuclei (AGNs) ( z ≲ 0.3), including their bolometric luminosity ( L _bol ), black hole (BH) mass ( M _BH ), accretion rates ( L _bol / L _Edd ), line-of-sight gas obscuration ( N _H ), and the distinctive properties of their host galaxies (e.g., star formation rates, masses, and gas fractions). We present an overview of the second data release of BASS (DR2), an unprecedented spectroscopic AGN survey in spectral range, resolution, and sensitivity, including 1449 optical (∼3200 Å–1 μ m) and 233 near-IR (1–2.5 μ m) spectra for the brightest 858 ultrahard X-ray (14–195 keV) selected AGNs across the entire sky and essentially all levels of obscuration. This release provides a highly complete set of key measurements (emission-line measurements and central velocity dispersions), with 99.9% measured redshifts and 98% BH masses estimated (for unbeamed AGNs outside the Galactic plane). The BASS DR2 AGN sample represents a unique census of nearby powerful AGNs, spanning over 5 orders of magnitude in AGN bolometric luminosity ( L _bol ∼ 10 ^40 –10 ^47 erg s ^−1 ), BH mass ( M _BH ∼ 10 ^5 –10 ^10 M _⊙ ), Eddington ratio ( L _bol / L _Edd ≳ 10 ^−5 ), and obscuration ( N _H ∼ 10 ^20 –10 ^25 cm ^−2 ). The public BASS DR2 sample and measurements can thus be used to answer fundamental questions about SMBH growth and its links to host galaxy evolution and feedback in the local universe, as well as open questions concerning SMBH physics. Here we provide a brief overview of the survey strategy, the key BASS DR2 measurements, data sets and catalogs, and scientific highlights from a series of DR2-based works pursued by the BASS team.

During a deep extragalactic Lockman Hole sky survey with an area of 18.5 sq. deg, which was conducted when the SRG observatory was flying to the Lagrange point L2, the eROSITA telescope detected X-ray sources. These objects were then provisionally identified and classified using the publicly accessible data of optical and infrared sky surveys by the SRGz machine learning system developed for this purpose at the Space Research Institute of the Russian Academy of Sciences. As a result, a number of new candidates for distant quasars () have been selected. The spectroscopic observations of the first two candidates from this list carried out with the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory have confirmed that these objects are actually distant quasars at redshifts 3.878 and 4.116 and are characterized by a high X-ray luminosity erg s (2–10 keV). The results obtained allow one to count on the detection of a large number of distant quasars during a four-year all-sky survey of the SRG observatory begun in December 2019. [ABSTRACT FROM AUTHOR]

We present the results of optical spectroscopy for 39 quasar candidates at photometric redshifts zphot ≳ 3 from the catalog by Khorunzhev et al. (2016) (K16). This is a catalog of quasar candidates and known type 1 quasars selected among the X-ray sources of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey. We have performed spectroscopy for new candidates at the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory and the 6-m BTA telescope of the Special Astrophysical Observatory. Predominantly X-ray bright quasar candidates that could be used to construct the X-ray luminosity function at luminosities LX,2–10 > 1045 erg s−1 have been selected. The purity of the candidates from K16 with a high X-ray flux has turned out to be lower than the purity of the quasi-random sample investigated by us previously. We have estimated the observing efficiency of quasar candidates as a function of their photometric redshift. [ABSTRACT FROM AUTHOR]

The X-ray luminosity function of distant (3 < z < 5.1) type 1 quasars has been measured. A sample of distant high-luminosity (1045 erg s−1 ≤ LX,2−10 < 7.5×1045 erg s−1 in the 2-10 keV energy band) quasars from the catalog by Khorunzhev et al. (2016) compiled from the data of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey and the Sloan Digital Sky Survey (SDSS) has been used. This sample consists of 101 sources. Most of them (90) have spectroscopic redshifts zspec ⩾ 3; the remaining ones are quasar candidates with photometric redshift estimates zphot ⩾ 3. The spectroscopic redshifts of eight sources have been measured with the BTA and AZT-33IK telescopes. Owing to the record sky coverage area (≃250 sq. deg at X-ray fluxes ~10−14 erg s−1 cm−2 in the 0.5-2 keVband) from which the sample was drawn, we have managed to obtain reliable estimates of the space density of distant X-ray quasars with luminosities LX,2−10 > 2×1045 erg s−1 for the first time. Their comoving space density remains constant as the redshift increases from z = 3 to 5 to within a factor of 2. The power-law slope of the X-ray luminosity function of distant quasars at its bright end (above the break) has been reliably constrained for the first time. The range of possible slopes for the quasar luminosity and density evolution model is γ2 = 2.72−0.12+0.19 ± 0.21, where initially the lower and upper boundaries of γ2 with the remaining uncertainty in the detection completeness of X-ray sources in SDSS and subsequently the statistical error of the slope are specified. [ABSTRACT FROM AUTHOR]

We have compiled a catalog of 903 quasar candidates (including known quasars) at 3 < z < 5.5 selected among X-ray sources from the XMM-Newton serendipitous survey (3XMM-DR4 catalog). We used photometric SDSS, 2MASS, and WISE data to select the objects. The surface number density of objects in our sample exceeds that in the SDSS spectroscopic quasar sample at the same redshifts by a factor of 1.5. We have performed spectroscopic observations of a subsample of new quasar candidates using a new low- and medium-resolution spectrograph at the 1.6-m AZT-33IK telescope (Mondy, Russia) and demonstrated that the purity of these candidates is about 65%. We have discovered one of the most distant (z = 5.08) X-ray selected quasars.

We present the results of optical spectroscopy for 19 quasar candidates at photometric redshifts z ≳ 3, 18 of which enter into the Khorunzhev et al. (2016) catalog (K16). This is a catalog of quasar candidates and known type 1 quasars selected among the X-ray sources of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey. We have performed spectroscopy for a quasi-random sample of new candidates at the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory and the 6-m BTA telescope of the Special Astrophysical Observatory. The spectra at AZT-33IK were taken with the new low- and medium-resolution ADAM spectrograph that was produced and installed on the telescope in 2015. Fourteen of the 18 candidates actually have turned out to be quasars; 10 of them are at spectroscopic redshifts z > 3. The high purity of the sample of new candidates suggests that the purity of the entire K16 catalog of quasars is probably 70-80%. One of the most distant ( z = 5.08) optically bright ( i′ ≲ 21) quasars ever detected in X-ray surveys has been discovered. [ABSTRACT FROM AUTHOR]

We have compiled a catalog of 903 candidates for type 1 quasars at redshifts 3 < z < 5.5 selected among the X-ray sources of the 'serendipitous' XMM-Newton survey presented in the 3XMMDR4 catalog (the median X-ray flux is ≈5 × 10 erg s cm in the 0.5-2 keV energy band) and located at high Galactic latitudes | b| > 20° in Sloan Digital Sky Survey (SDSS) fields with a total area of about 300 deg. Photometric SDSS data as well infrared 2MASS and WISE data were used to select the objects. We selected the point sources from the photometric SDSS catalog with a magnitude error δ′ < 0.2 and a color i′ − z′ < 0.6 (to first eliminate the M-type stars). For the selected sources, we have calculated the dependences χ( z) for various spectral templates from the library that we compiled for these purposes using the EAZY software. Based on these data, we have rejected the objects whose spectral energy distributions are better described by the templates of stars at z = 0 and obtained a sample of quasars with photometric redshift estimates 2.75 < z < 5.5. The selection completeness of known quasars at z > 3 in the investigated fields is shown to be about 80%. The normalized median absolute deviation (Δ z = | z − z|) is σ/(1+ z) = 0.07, while the outlier fraction is η = 9% when Δ z/(1 + z.) > 0.2. The number of objects per unit area in our sample exceeds the number of quasars in the spectroscopic SDSS sample at the same redshifts approximately by a factor of 1.5. The subsequent spectroscopic testing of the redshifts of our selected candidates for quasars at 3 < z < 5.5 will allow the purity of this sample to be estimated more accurately. [ABSTRACT FROM AUTHOR]

A sample of cataclysmic variables (CVs) detected among the X-ray sources of the 400 square degree (400d) survey performed based on ROSAT pointings is presented. A technique for selecting CVs among the X-ray sources using additional optical and infrared data, based on Sloan Digital Sky Survey andWISE data, is described. We present the optical observations of the selected objects carried out mainly with the Russian-Turkish 1.5-m telescope (RTT-150) and the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (BTA). Some observations have also been performed with the 1.6-m AZT-33IK telescope of the Institute of Solar-Terrestrial Physics, the Siberian branch of the Russian Academy of Sciences. Eight CVs, four of which were detected in our work, have been selected by now. Based on this sample, we have obtained preliminary constraints on the X-ray luminosity function of CVs in the solar neighborhood in the range of low luminosities, L ~ 10-10 erg s (0.5-2 keV). The logarithmic slope of the CV luminosity function in this luminosity range is shown to become gentler than that at L > 10 erg s. It follows from our estimates of the CV luminosity function that several thousand CVs will be detected in the SRG all-sky survey at high Galactic latitudes, which will allow much more accurate measurements of their X-ray luminosity function to be obtained. [ABSTRACT FROM AUTHOR]

We analyze the statistical properties of normal galaxies to be detected in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory. With the current configuration and parameters of the eROSITA telescope, the sensitivity of a 4-year-long all-sky survey will be ≈10−14 erg s−1 in the 0.5–2 keV band. This will allow ∼(1.5–2) × 104 normal galaxies with approximately the same contribution of star-forming and elliptical galaxies to be detected. All galaxies of the X-ray survey are expected to enter into the existing far-infrared (IRAS) or near-infrared (2MASS) catalogs; the sample of star-forming galaxies will be approximately equivalent in sensitivity to the sample of star-forming galaxies in the IRAS catalog of infrared sources. Thus, a large homogeneous sample of normal galaxies with measured X-ray, near-infrared, and far-infrared fluxes will be formed. About 90% of the galaxies in the survey are located within ∼200–400 Mpc. A typical (most probable) galaxy will have a luminosity log LX ∼ 40.5–41.0, will be located at a distance of ∼70–90 Mpc, and will be either a star-forming galaxy with a star formation rate of ∼20 M⊙ yr−1 whose X-ray emission is produced by ultraluminous X-ray sources (ULXs) or an elliptical galaxy with amass log M* ∼ 11.3 emitting through to a hot interstellar gas. The galaxies within 35 Mpc will collectively contain ∼102 ULXs with luminosities log LX > 40, ∼80% of whichwill be the only luminous source in the galaxy. Thus, although the angular resolution of the eROSITA telescope is too low for the luminosity function of compact sources in galaxies to be studied in detail, the survey data will allow one to investigate its bright end and, possibly, to impose constraints on the maximum luminosity of ULXs. [ABSTRACT FROM AUTHOR]