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The International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, continues its successful work in observing the sky at energies E>20 keV. The legacy of the mission already includes a large number of discovered or previously poorly studied hard X-ray sources. The growing INTEGRAL archive allows one to conduct an all-sky survey including a number of deep extragalactic fields and the deepest ever hard X-ray survey of the Galaxy. Taking advantage of the data gathered over 17 years with the IBIS coded-mask telescope of INTEGRAL, we conducted survey of hard X-ray sources, providing flux information from 17 to 290 keV. The catalog includes 929 objects, 890 of which exceed a detection threshold of 4.5 sigma and the rest are detected at 4.0-4.5 sigma and belong to known cataloged hard X-ray sources. Among the identified sources of known or suspected nature, 376 are associated with the Galaxy and Magellanic clouds, including 145 low-mass and 115 high-mass X-ray binaries, 79 cataclysmic variables, and 37 of other types; and 440 are extragalactic, including 429 active galactic nuclei (AGNs), 2 ultra-luminous sources, one supernova (AT2018cow) and 8 galaxy clusters. 113 sources remain unclassified. 46 objects are detected in the hard X-ray band for the first time. The LogN-LogS distribution of 356 non-blazar AGNs is measured down to a flux of 2E-12 erg/s/cm2 and can be described by a power law with a slope of 1.44 +/- 0.09 and normalization 8E-3/deg2 at 1E-11 erg/s/cm2. The LogN-LogS distribution of unclassified sources indicates that the majority of them are of extragalactic origin., 5 pages, 2 figures, 7 tables (1 long table), accepted for publication in MNRAS

The models and results of the experimental studies of the characteristics of objects that are new for domestic instrumentation are presented: an X-ray mirror system and a CdTe strip semiconductor detector that are used in the first Russian ART-XC reflecting X-ray telescope. This telescope is a part of the Spektr-RG international space observatory, which started to the L2 libration point in July 2019.

The results of the broadband spectral and timing study of the recently discovered transient X-ray pulsar MAXI J0903-531 in a wide range of luminosities differing by a factor of ~30 are reported. The observed X-ray spectrum in both states can be described as a classical pulsar-like spectrum consisting of the power-law with the high-energy cutoff. We argue that absence of the spectrum transformation to the two-hump structure expected at low fluxes points to a relatively weak magnetic field of the neutron star below (2-3)$\times10^{12}$ G. This estimate is consistent with other indirect constraints and non-detection of any absorption features which can be interpreted as a cyclotron absorption line. Timing analysis of the NuSTAR data revealed only slight variations of a single-peaked pulse profile of the source as a function of the energy band and mass accretion rate. In both intensity states the pulsed fraction increases from 40% to roughly 80% with the energy. Finally we were also able to obtain the orbital solution for the binary system using data from the Fermi/GBM, NICER and NuSTAR instruments., Accepted to A&A for publication in the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission. 8 pages, 6 figures

We have performed a joint spectral and timing analysis of the outburst of GRS 1739-278 in 2014 based on Swift and INTEGRAL data. We show that during this outburst the system exhibited both intermediate states: hard and soft. Peaks of quasi-periodic oscillations (QPOs) in the frequency range 0.1-5 Hz classified as type-C QPOs have been detected from the system. Using Swift/BAT data we show that after the 2014 outburst the system passed to the regime of mini-outburst activity: apart from the three mini-outbursts mentioned in the literature, we have detected four more mini-outbursts with a comparable (~20 mCrab) flux in the hard energy band (15-50 keV). We have investigated the influence of the accretion history on the outburst characteristics: the dependence of the peak flux in the hard energy band in the low/hard state on the time interval between the current and previous peaks has been found (for the outbursts during which the system passed to the high/soft state)., 19 pages, 10 figures

The possibility of conducting a Galactic plane survey at energies above 5 keV with the ART-XC telescope onboard the Spectrum-Rontgen-Gamma (SRG) observatory during the satellite’s flight to the Lagrange point L2 is considered. One of the possible fields for such a survey is proposed. We show that it will be possible to detect ∼100 cataclysmic variables with luminosities 1031−1034 erg s−1 in a survey with an area of 10 deg2 and a duration of 10 days. Such a representative sample may allow the luminosity function and other properties of the population of cataclysmic variables in the Galaxy to be refined.

From October 2016 to September 2017, we performed tests of the ART-XC /SRG spare mirror system and detector unit at the 60-m-long IKI X-ray test facility. We describe some technical features of this test facility. We also present a brief description of the ART-XC mirror system and focal detectors. The nominal focal length of the ART-XC optics is 2700 mm. The field of view is determined by the combination of the mirror system and the detector unit and is equal to ∼0.31 square degrees. The declared operating energy range is 5–30 keV. During the tests, we illuminated the detector with a 55Fe+241 Am calibration source and also with a quasi-parallel X-ray beam. The calibration source is integrated into the detector’s collimator. The X-ray beam was generated by a set of Oxford Instruments X-ray tubes with Cr, Cu and Mo targets and an Amptek miniature X-ray tube (Mini-X) with Ag transmission target. The detector was exposed to the X-ray beam either directly or through the mirror system. We present the obtained results on the detector’s energy resolution, the muon on-ground background level and the energy dependence of the W90 value. The accuracy of a mathematical model of the ART-XC mirror system, based on ray-tracing simulations, proves to be within 3.5% in the main energy range of 4–20 keV and 5.4% in the “hard” energy range of 20–40 keV.

ART-XC (Astronomical Roentgen Telescope - X-ray Concentrator) is the hard X-ray instrument with grazing incidence imaging optics on board the Spektr-Roentgen-Gamma (SRG) observatory. The SRG observatory is the flagship astrophysical mission of the Russian Federal Space Program, which was successively launched into orbit around the second Lagrangian point (L2) of the Earth-Sun system with a Proton rocket from the Baikonur cosmodrome on 13 July 2019. The ART-XC telescope will provide the first ever true imaging all-sky survey performed with grazing incidence optics in the 4-30 keV energy band and will obtain the deepest and sharpest map of the sky in the energy range of 4-12 keV. Observations performed during the early calibration and performance verification phase as well as during the on-going all-sky survey that started on 12 Dec. 2019 have demonstrated that the in-flight characteristics of the ART-XC telescope are very close to expectations based on the results of ground calibrations. Upon completion of its 4-year all-sky survey, ART-XC is expected to detect ~5000 sources (~3000 active galactic nuclei, including heavily obscured ones, several hundred clusters of galaxies, ~1000 cataclysmic variables and other Galactic sources), and to provide a high-quality map of the Galactic background emission in the 4-12 keV energy band. ART-XC is also well suited for discovering transient X-ray sources. In this paper, we describe the telescope, results of its ground calibrations, major aspects of the mission, the in-flight performance of ART-XC and first scientific results., 19 pages, 30 figures, accepted for publication in Astronomy and Astrophysics

We report on observations of the Be/X-ray binary system SwiftJ1626.6-5156 performed with NuSTAR during a short outburst in March 2021, following its detection of by the MAXI monitor and Spektrum-Roentgen-Gamma (SRG) observatory. Our analysis of the broadband X-ray spectrum of the source confirms the presence of two absorption-like features at energies ~9 and ~17 keV previously reported in literature and interpreted as the fundamental cyclotron resonance scattering feature (CRSF) and its first harmonic (based on RXTE data). The better sensitivity and energy resolution of NuSTAR, combined with the low energy coverage of NICER, allowed us to detect two additional absorption-like features at ~4.9 keV and ~13 keV. We conclude, therefore, that in total four cyclotron lines are observed in the spectrum of SwiftJ1626.6-5156: the fundamental CRSF at ~4.9 keV and three higher spaced harmonics. This discovery makes SwiftJ1626.6-5156 the second accreting pulsar, after 4U0115+63, whose spectrum is characterized by more than three lines of a cyclotronic origin, and implies the source has the weakest confirmed magnetic field among all X-ray pulsars B~4E11 G. This discovery makes SwiftJ1626.6-5156 one of prime targets for the upcoming X-ray polarimetry missions covering soft X-ray band such as IXPE and eXTP.

The INTEGRAL hard X-ray surveys have proven to be of fundamental importance. INTEGRAL has mapped the Galactic plane with its large field of view and excellent sensitivity. Such hard X-ray snapshots of the whole Milky Way on a time scale of a year are beyond the capabilities of past and current narrow-FOV grazing incidence X-ray telescopes. By expanding the INTEGRAL X-ray survey into shorter timescales, a productive search for transient X-ray emitters was made possible. In more than fifteen years of operation, the INTEGRAL observatory has given us a sharper view of the hard X-ray sky, and provided the triggers for many follow-up campaigns from radio frequencies to gamma-rays. In addition to conducting a census of hard X-ray sources across the entire sky, INTEGRAL has carried out, through Earth occultation maneuvers, unique observations of the large-scale cosmic X-ray background, which will without question be included in the annals of X-ray astronomy as one of the mission's most salient contribution to our understanding of the hard X-ray sky., Comment: 23 pages, 16 figures, 1 table, will be published in New Astronomy Reviews issue 15 years of INTEGRAL science

Based on long-term SWIFT, RXTE, and MAXI observations of the X-ray novae H1743-322 (IGR J17464-3213) and GX339-4, we have investigated the morphology and classified the light curves of their X-ray outbursts. In particular, we have confirmed the existence of two radically different types of outbursts, soft (S) and hard (H), in both sources and revealed their varieties, ultrabright (U) and intermediate (I). The properties and origin of the differences in the light curves of these outbursts are discussed in terms of the truncated accretion disk model., 19 pages, 14 figures, 2 tables

AGN are among the most energetic phenomena in the Universe and in the last two decades INTEGRAL's contribution in their study has had a significant impact. Thanks to the INTEGRAL extragalactic sky surveys, all classes of soft X-ray detected (in the 2-10 keV band) AGN have been observed at higher energies as well. Up to now, around 450 AGN have been catalogued and a conspicuous part of them are either objects observed at high-energies for the first time or newly discovered AGN. The high-energy domain (20-200 keV) represents an important window for spectral studies of AGN and it is also the most appropriate for AGN population studies, since it is almost unbiased against obscuration and therefore free of the limitations which affect surveys at other frequencies. Over the years, INTEGRAL data have allowed to characterise AGN spectra at high energies, to investigate their absorption properties, to test the AGN unification scheme and to perform population studies. In this review the main results are reported and INTEGRAL's contribution to AGN science is highlighted for each class of AGN. Finally, new perspectives are provided, connecting INTEGRAL's science with that at other wavelengths and in particular to the GeV/TeV regime which is still poorly explored., Comment: 31 pages, 16 figures, will be published in New Astronomy Reviews issue 15 years of Integral science

We report a discovery of low-frequency quasi periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS1739-278 in the hard-intermediate state during its 2014 outburst based on the ${\it NuSTAR}$ and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase-lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be $R_{\rm in} < 7.3 R_{\rm g}$. In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with $M_{\rm BH} \approx 100$M$_\odot$ is required., 15 pages, 12 figures; accepted for publication in MNRAS

To choose the best strategy for conducting a deep extragalactic survey with the ART-XC X-ray telescope onboard the Spectrum–Rontgen–Gamma (SRG) observatory and to estimate the expected results, we have simulated the observations of a 1.1° × 1.1° field in the 5–11 and 8–24 keV energy bands. For this purpose, we have constructed a model of the active galactic nuclei (AGN) population that reflects the properties of the X-ray emission from such objects. The photons that “arrived” from these sources were passed through a numerical model of the telescope, while the resulting data were processed with the standard ART-XC data processing pipeline. We show that several hundred AGNs at redshifts up to z ≈ 3 will be detected in such a survey over 1.2 Ms of observations with the expected charged particle background levels. Among them there will be heavily obscured AGNs, which will allow a more accurate estimate of the fraction of such objects in the total population to be made. Source confusion is expected at fluxes below 2 × 10−14 erg s−1 cm−2 (5–11 keV). Since this value can exceed the source detection threshold in a deep survey at low particle background levels, it may turn out to be more interesting to conduct a survey of larger area (several square degrees) but smaller depth, obtaining a sample of approximately four hundred bright AGNs as a result.

We present the results of our analysis of the JEM-X/INTEGRAL data obtained from January 2003 to January 2015 aimed at searching for type I X-ray bursts from known and new bursters. Such bursts are caused by thermonuclear explosions on the surface of a neutron star. We have searched for bursts in the records of the count rate of the JEM-X detectors in the 3–20 keV energy band. We have separately reconstructed and analyzed the light curves of 104 X-ray bursters known to date based on the JEM-X data. A similar search for bursts was previously carried out in the 15–25 keV data from the IBIS/ISGRI telescope onboard the INTEGRAL observatory obtained in 2003–2009. We have continued to analyze the data from this telescope up until the observations in January 2015. The joint catalog of bursts detected by the two instruments includes 2201 events; their basic parameters are given. The large size of the sample of bursts makes it one of the most representative of the existing one and allows various statistical studies of bursts to be performed. In particular, we have constructed the dependence of the mean rate of type I bursts from bursters on the luminosity (accretion rate), revealed an appreciable burst rate from sources with a near-Eddington luminosity, and investigated the population of multiple bursts with a recurrence time much shorter than the time it takes for a critical mass of matter required for the initiation of an explosion to be accumulated on the neutron star surface. Almost all of the detected bursts are associated with already known bursters,we have found only one previously unknown burster, IGRJ17380-3749, in the archival data, and one more known, but poorly studied source, AX J1754.2-2754, has been identified as a burster. Several similar sources have previously been identified as bursters directly during the INTEGRAL observations.

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) continues to successfully work in orbit after its launch in 2002. The mission provides the deepest ever survey of hard X-ray sources throughout the Galaxy at energies above 20 keV. We report on a catalogue of new hard X-ray source candidates based on the latest sky maps comprising 14 years of data acquired with the IBIS telescope onboard INTEGRAL in the Galactic Plane (|b|4.7 sigma and not known to previous INTEGRAL surveys. Among them, 31 objects have also been detected in the on-going all-sky survey by the BAT telescope of the Swift observatory. For 26 sources on the list, we suggest possible identifications: 21 active galactic nuclei, two cataclysmic variables, two isolated pulsars or pulsar wind nebulae, and one supernova remnant; 46 sources from the catalogue remain unclassified., 7 pages, 2 figures, 2 tables. Submitted to MNRAS Letters, comments welcome

We report the discovery of absorption features in the X-ray spectrum of the transient X-ray pulsar GROJ2058+42. The features are detected around $\sim10$, $\sim20$ and $\sim30$ keV in both NuSTAR observations carried out during the source type II outburst in spring 2019. The most intriguing property is that the deficit of photons around these energies is registered only in the narrow phase interval covering around 10% of the pulsar spin period. We interpret these absorption lines as a cyclotron resonant scattering line (fundamental) and two higher harmonics. The measured energy allow us to estimate the magnetic field strength of the neutron star as $\sim10^{12}$ G., 7 pages, 5 figures, accepted to ApJL

We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasi-periodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field $B \lesssim 2 \times10^{12}$ G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field $B < 6 \times 10^{11}$ G., 10 pages, 7 figures, submitted to MNRAS

An X-ray survey of the Galactic center region with a radius of ~20◦ has been performed using the data obtained with the JEM-X telescope onboard the INTEGRAL observatory over ~10 years of observations (2003–2013). The exposure at the field center directly toward the Galactic center has reached 4.8 Ms. We have constructed sky maps in the 5–10 and 10–25 keV energy bands and compiled a catalog of detected sources. Together with 83 sources revealed on the integral sky maps, it includes 22 transients that are absent on them but are confidently detected during outbursts with a duration of several days. One of the persistent sources, IGR J17452-2909, has never been observed previously. In contrast to the catalog of sources detected in the harder energy band by the IBIS/ISGRI telescope onboard the INTEGRAL observatory, most of the sources in this catalog are low-mass X-ray binaries (73 of the 105 sources) and only 18 + 3 are high-mass X-ray binaries and cataclysmic variables. Out of the Galactic sources, there are also the black hole candidate XTE J1652-453, the peculiar X-ray burster XMMJ174457-2850.3, and the soft gamma repeater SGR 1806–20 in the catalog; out of the extragalactic sources, there are three active galactic nuclei and a galaxy cluster (Oph CL). The nature of four sources, including the newly discovered one, still remains unknown. We have constructed the luminosity function for the low-mass X-ray binaries from the catalog and considered other statistical properties of their sample.

IGR J17591−2342 is an accreting millisecond X-ray pulsar, discovered with INTEGRAL, which went into outburst around July 21, 2018. To better understand the physics acting in these systems during the outburst episode, we performed detailed temporal-, timing-, and spectral analyses across the 0.3–300 keV band using data from NICER,XMM-Newton,NuSTAR, and INTEGRAL. The hard X-ray 20–60 keV outburst profile covering ∼85 days is composed of four flares. Over the course of the maximum of the last flare, we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data, posing constraints on the source distance. We derived a distance of 7.6 ± 0.7 kpc, adopting Eddington-limited photospheric radius expansion and assuming anisotropic emission. In the timing analysis, using all NICER 1–10 keV monitoring data, we observed a rather complex set of behaviours starting with a spin-up period (MJD 58345–58364), followed by a frequency drop (MJD 58364–58370), an episode of constant frequency (MJD 58370–58383), concluded by irregular behaviour till the end of the outburst. The 1–50 keV phase distributions of the pulsed emission, detected up to ∼120 keV using INTEGRAL ISGRI data, was decomposed in three Fourier harmonics showing that the pulsed fraction of the fundamental increases from ∼10% to ∼17% going from ∼1.5 to ∼4 keV, while the harder photons arrive earlier than the soft photons for energies ≲10 keV. The total emission spectrum of IGR J17591−2342 across the 0.3–150 keV band could adequately be fitted in terms of an absorbedCOMPPS model yielding as best fit parameters a column density ofNH = (2.09 ± 0.05) × 1022cm−2, a blackbody seed photon temperaturekTbb, seedof 0.64 ± 0.02 keV, electron temperaturekTe = 38.8 ± 1.2 keV and Thomson optical depthτT = 1.59 ± 0.04. The fit normalisation results in an emission area radius of 11.3 ± 0.5 km adopting a distance of 7.6 kpc. Finally, the results are discussed within the framework of accretion physics- and X-ray thermonuclear burst theory.

Results of the deep survey of the Large Magellanic Cloud (LMC), performed with the INTEGRAL observatory, are presented. The long exposure (~7 Ms) allowed us to detect twenty one sources in this sky region: ten belonging to the LMC itself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and three belonging to other galaxies from the Local Group - the Milky Way (2 sources) and Small Magellanic Cloud (1 source). Four new hard X-ray sources of these 21 ones were discovered during the survey in addition to IGR J05414-6858 reported earlier; two of them were identified with extragalactic objects. We report also for the first time the detection of a hard X-ray emission from the Crab-like pulsar PSR J0537-6910 and identification of the hard X-ray source IGR J05305-6559 with the high-mass X-ray binary EXO 053109-6609., Comment: 9 pages, 6 figures, accepted to MNRAS

We present results of deep surveys of three extragalactic fields, M81 (exposure of 9.7 Ms), LMC (6.8 Ms) and 3C 273/Coma (9.3 Ms), in the hard X-ray (17-60 keV) energy band with the IBIS telescope onboard the INTEGRAL observatory, based on 12 years of observations (2003-2015). The combined survey reaches a $4\sigma$ peak sensitivity of 0.18 mCrab (2.6$\times$10$^{-12}$ erg s$^{-1}$ cm$^{-2}$) and sensitivity better than 0.25 and 0.87 mCrab over 10% and 90% of its full area of 4900 deg$^{2}$, respectively. We have detected in total 147 sources at $S/N>4\sigma$, including 37 sources observed in hard X-rays for the first time. The survey is dominated by extragalactic sources, mostly by active galactic nuclei (AGN). The sample of identified sources contains 98 AGN (including 64 Seyfert galaxies, 7 LINERs, 3 XBONGs, 16 blazars and 8 AGN of unclear optical class), two galaxy clusters (Coma and Abell 3266), 17 objects located in the Large and Small Magellanic Clouds (13 high- and 2 low-mass X-ray binaries and 2 X-ray pulsars), three Galactic cataclysmic variables, one ultraluminous X-ray source (ULX, M82 X-1) and one blended source (SWIFT J1105.7+5854). The nature of 25 sources remains unknown, so that the surveys identification is currently complete at 83%. We have constructed AGN number-flux relations ($\log{N}$-$\log{S}$) and calculated AGN number densities in the local Universe for the entire survey and for each of the three extragalactic fields., Comment: 11 pages, 10 figures, submitted to MNRAS