We present X-ray and Optical/IR observational analysis for J163331.6-445727 - a newly discovered Symbiotic X-ray Binary (SyXB) system in the far Norma Arm. In our search for new X-ray binaries (XBs) in the 4XMM catalog J163331.6-445727 was selected among other XB candidates using machine learning technique. To establish its nature we analyzed archival data of XMM-Newton and carried out a dedicated pointed X-ray observation with the Mikhail Pavlinsky ART-XC telescope on board the SRG mission and optical spectroscopic observations with the Robert Stobie Spectrograph on SALT. X-ray data allowed us to reveal in J163331.6-445727 coherent X-ray pulsations with the period of similar or equal to 1552 s. The optical counterpart is a reddened (A(V) approximate to 5.5(mag)) giant star of the K3-3.5III spectral type at a distance of approximate to 6.5 kpc. According to the presence of the pulsations, hardness of X-ray spectrum and optical companion type, we conclude that J163331.6-445727 belongs to a rare class of distant SyXB systems.

This article examines the calibration of the M.N. Pavlinsky ART-XC X-ray space telescope, which has been operating in space since 2019. This is the first X-ray mirror telescope in Russia. The novelty of this telescope required the development of new methods and equipment for its calibration. The methodology, procedure, and results of the telescope calibration and a test bench for this purpose are described. The results of processing the telescope observations of some astrophysical sources obtained on the basis of the performed calibration are presented.

After several decades of relative quiescence, the compact steep-spectrum quasar 3C 138 entered an active phase in 2024-2026, exhibiting strong broadband flaring. We investigate its multiwavelength behaviour using dense multifrequency radio monitoring at 1-22 GHz with RATAN-600 and RT-32, optical R-band observations with Zeiss-1000 and AS-500/2, X-ray measurements with Swift/XRT and SRG/ART-XC, and the Fermi-LAT γ-ray light curve. The radio brightening accelerated after 2022 and was strongest at the highest frequencies. The radio spectra hardened markedly, with the 11-22 GHz spectral index evolving from steep to flat or inverted during the active phase. The X-ray flux increased by more than a factor of three during 2025-2026, while the photon index hardened from Γ_ X≃ 1.6 to Γ_ X≃ 0.9 and softened back after the peak. Flare decomposition revealed five γ-ray flares and a sequence of optical subflares during the later stages of the activity. The γ-ray, X-ray, and optical maxima occur within a ≃ 13-day interval, suggesting a common activity episode, whereas the radio brightens more gradually and in a frequency-dependent manner. Under the adopted compact-zone geometries, the sparse two-state spectral energy distributions (SEDs) can be represented by one-zone synchrotron self-Compton (SSC) solutions, while the relative contribution of external Compton (EC) remains geometry dependent. The flare shifts the modelled energy partition towards relativistic electrons. These results favour a longer-lived, core-dominated activity phase, with later high-energy and optical flares superposed on the opacity-driven radio evolution of an emerging synchrotron component.

We report on new observations of a historical hard X-ray transient Swift J2037.2+4151. Utilising new Chandra data, we improved on the localisation of the source, confirming its association with the bright infrared source SSTSL2 J203705.58+415005.3. Based on the near-infrared spectrum and broadband spectral energy distribution, we argue that the optical star in the system is a red giant with Teff ≈ 3800 K, located at D > 5 kpc. Given the nature of the optical star, we confirm that the Swift J2037.2+4151 is another symbiotic X-ray binary residing in the Galactic plane. An analysis of X-ray data, gathered in 2022–2025, shows that the source resides in a low state. This is caused by simultaneous decrease in the accretion rate and a significant (of an order of magnitude) increase in the thickness of the absorption column.

Using the Mikhail Pavlinsky ART-XC onboard the SRG observatory we have detected, for the first time, X-ray pulsations with a period of 106 s from the poorly-studied high-mass X-ray binary RX J0535.0-6700 located in the Large Magellanic Cloud (LMC), thus proving that the accretor is a neutron star with strong magnetic field. Pulsations with similar period were also found in archival archival data from Chandra and XMM-Newton telescopes. Using photometry from WISE we shown that the source demonstrate significant variability in IR during the last twenty years, which could be caused by a secular evolution of the decretion disk. This discovery makes RX J0535.0-6700 another member of the large family of X-ray pulsars with Be-type companions in the LMC.

Long uninterrupted observations of the X-ray binary system Her X-1 were performed with the Mikhail Pavlinsky ART-XC telescope of the Spectrum-R & ouml;ntgen-Gamma (SRG) X-ray Observatory in the 4-25 keV energy range with total exposure of about two days around the main turn-on of the X-ray source. We present the results of the timing and spectral analysis of these observations. The opening of the X-ray source is determined to occur at the orbital phase phi(b) approximate to 0.25. The analysis of the X-ray light curve reveals the first direct observational evidence of the nutation of a tilted precessing accretion disk with a period of similar or equal to 0.87 days. The appearance of X-ray pulsations near the orbital phase phi(b) similar or equal to 0.77 prior to the main turn-on at the maximum of the nutation variability has also been detected. During the X-ray eclipse, a non-zero X-ray flux is measured, which is presumably associated with the scattering of X-ray emission in a hot corona around the optical star illuminated by the X-rays from the central neutron star. An increase in the X-ray flux after the main turn-on can be described by the passage of radiation from the central source through a scattering corona above the precessing accretion disk.

We present a catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope onboard the SRG space observatory during the observations of the Galactic plane region near a longitude l≃ 20^∘ (L20 field) in October 2019. The L20 field was observed four times in the scanning mode, which provided a uniform coverage of the sky region with a total area of ≃24 deg^2 with a median sensitivity of 8× 10^-13 erg s ^-1 cm ^-2 (at 50 % detection completeness) in the 4–12 keV energy band. As a result, we have detected 29 X-ray sources at a statistically significant level, 11 of which have not been detected previously by other X-ray observatories. Preliminary estimates show that four of them can presumably be extragalactic in nature. We also show that the source SRGA J183220.1 - 103508 (CXOGSG J183220.8 - 103510) is most likely a galaxy cluster containing a bright radio galaxy at redshift z≃ 0.121 .

Dark matter sterile neutrinos radiatively decay in the Milky Way, which can be tested with searches for almost monochromatic photons in the X-ray cosmic spectrum. We analyse the data of SRG/ART-XC telescope operated for two years in the all-sky survey mode. With no significant hints in the Galactic diffuse X-ray spectrum we explore models with sterile neutrino masses in 12-40 keV range and exclude corresponding regions of sterile-active neutrino mixing.

We report the discovery of the new accreting millisecond X-ray pulsar SRGA J144459.2-604207 using data of the SRG/ART-XC. The source was observed twice in February 2024 during the declining phase of the outburst. The timing analysis revealed a coherent signal near 447.9 Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with a circular orbit with a period of similar to 5.2 h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can be well modeled by emission from two circular spots that are partially eclipsed by the accretion disk. Additionally, during our observations with an exposure of 133 ks, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and none show any signs of an expanding photospheric radius. The burst recurrence times decreases linearly from similar to 1.6 h at the beginning of observations to similar to 2.2 h at the end and anticorrelate with the persistent flux. The spectral evolution during the bursts is consistent with the models of the neutron star atmospheres that are heated by accretion and implies a neutron star radius of 11-12 km and a distance to the source of 8-9 kpc. We also detected coherent pulsations during the bursts and showed that the pulse profiles differ substantially from those observed in the persistent emission. However, we could not find a simple physical model explaining the pulse profiles detected during the bursts.

— The achieved results in the field of coherent optical frequency reflectometry (COFR) based on a source of frequency-tunable radiation of a new type—a fiber self-scanning laser—are reviewed. It was shown that the developed COFR can be used for the tasks of characterization of optical elements, sensors, vibration measurement, and gas analysis at lengths of several tens of meters with submillimeter spatial resolution. Prospects for the development of this trend are also discussed.

We present a catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope onboard the SRG space observatory during the observations of the Galactic plane region near a longitude $$l\simeq 20^{\circ}$$ (L20 field) in October 2019. The L20 field was observed four times in the scanning mode, which provided a uniform coverage of the sky region with a total area of $${\simeq}24\text{ deg}^{2}$$ with a median sensitivity of $$8\times 10^{-13}$$ erg s $${}^{-1}$$ cm $${}^{-2}$$ (at 50 $$\%$$ detection completeness) in the 4–12 keV energy band. As a result, we have detected 29 X-ray sources at a statistically significant level, 11 of which have not been detected previously by other X-ray observatories. Preliminary estimates show that four of them can presumably be extragalactic in nature. We also show that the source SRGA J183220.1 $$-$$ 103508 (CXOGSG J183220.8 $$-$$ 103510) is most likely a galaxy cluster containing a bright radio galaxy at redshift $$z\simeq 0.121$$ .

We report the possibility of measuring the temperature distribution along a conventional single-mode fiber by coherent optical frequency reflectometry using a self-sweeping fiber laser and Rayleigh scattering of light by inhomogeneities frozen into the fiber. To this end, the problem of increasing the sensitivity of the reflectometer to a level of about –120 dB mm–1 is solved. A linear relationship is shown between a change in fiber temperature and a shift in the reflection spectrum of 4-cm long heated fiber section with a sensitivity of 2 GHz C°–1.

Context. During its ongoing all-sky survey, the Mikhail Pavlinsky ART-XC (Astronomical Roentgen Telescope - X-ray Concentrator) telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory is set to discover new X-ray sources, many of which can be transient. Here we report the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226 = SRGe J043523.3+552234. This is the high-energy counterpart of the optical transient AT2019wey. Aims. Through its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using the synergy with current public optical surveys, we aim to reveal the nature of these transients to study their parent populations. SRGA J043520.9+552226 is the first transient detected by ART-XC that has a bright optical counterpart suitable for further studies. Methods. We used available public X-ray and optical data and observations with SRG, INTEGRAL, NuSTAR, NICER, Swift, and ground-based telescopes to investigate the spectral energy distributions of the source in different phases of the outburst. Results. Based on X-ray spectral and timing properties derived from space observations, optical spectroscopy, and photometry obtained with the 2.5 m and RC600 telescopes of the Caucasian Mountain Observatory of the Sternberg Astronomical Institute of Moscow State University, we propose that the source is a black hole in a low-mass close X-ray binary system.

We report a discovery of a new long-period X-ray pulsar SRGA J204318.2+443815/SRGe J204319.0+443820 in the Be binary system. The source was found in the second all-sky survey by the Mikhail Pavlinsky telescope on board the SRG mission. The follow-up observations with XMM-Newton, NICER and NuSTAR observatories allowed us to discover a strong coherent signal in the source light curve with the period of $\sim742$ s. The pulsed fraction was found to depend on the energy increasing from $\sim20$% in soft X-rays to $>50$% at high energies, as it is typical for X-ray pulsars. The source demonstrate a quite hard spectrum with an exponential cutoff at high energies and bolometric luminosity of $L_X \simeq 4\times10^{35}$ erg/s. Dedicated optical and infrared observations with the RTT-150, NOT, Keck and Palomar telescopes revealed a number of emission lines (H$_{\alpha}$, HeI, Pashen and Braket series) with the strongly absorbed continuum. All of above suggests that SRGAJ204318.2+443815/ SRGeJ204319.0+443820 is a new persistent low luminosity X-ray pulsar in a distant binary system with a Be-star of the B0-B2e class. Thus the SRG observatory allow us to unveil the hidden population of faint persistent objects including the population of slowly rotating X-ray pulsars in Be systems.

Close binary systems consisting of two neutron stars (BNS) emit gravitational waves, that allow them to merge on timescales shorter than Hubble time. It is widely believed, that NS–NS mergers in such systems power short gamma-ray bursts (GRB). Several mechanisms which could lead to electromagnetic energy release prior to a merger have been proposed. We estimate the ability to observe the possible pre-burst emission with telescopes of Spectrum–Roentgen–Gamma. We also investigate first such event, GRB210919A, which fell into the field of view of the SRG telescopes less than two days before the burst.

A module has been designed for automated acquisition and preliminary processing of optical signals from devices based on a fiber laser with frequency self-sweeping. The primary processing, i.e., pulse-by-pulse data normalization, is performed using a microcontroller. Signals are then transmitted to a personal computer for further processing in the form of the dependence of the signal amplitude on the pulse number. The optimal characteristics of the used analog-to-digital converter are as follows: the sampling rate is 5 MHz and the bit depth is at least 8 bits. The operation of the module was tested in an optical frequency reflectometer for interrogating fiber sensors.

The first results are presented that demonstrate the applicability of a coherent optical frequency-domain reflectometer based on a self-sweeping fiber laser for sensing. An array of various fiber Bragg gratings (FBGs) was used as the sensing line. It is shown that FBGs with a reflection spectrum that lies outside the scan region of ​​our source can be used in the developed reflectometer. Using the frequency-domain reflectometry technique, it is possible to perform spatial separation of FBG sensors, even with the use of the same reflection wavelengths. The possibility of measuring the temperature has been experimentally demonstrated.

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 first results of an experimental study of a coherent optical frequency-domain reflectometer (C-OFDR) based on a simple single-frequency fibre laser with frequency self-scanning are presented. The self-scanning laser generates microsecond pulses, and its lasing frequency changes linearly with a change in the pulse number without using any actively tuned elements. In addition, the generation of each pulse occurs on only one longitudinal mode with a linewidth less than 1 MHz. This laser is characterised by a high linearity of frequency tuning, due to which reflectograms can be measured without any additional spectral correction. The C-OFDR demonstrates a possibility of attaining a spatial sampling of ∼200 μm and a reflectance sensitivity of approximately down to – 80 dB at a test line length of ∼9 m.

The possibility of conducting a Galactic plane survey at energies above 5 keV with the ART-XC telescope onboard the Spectrum-Röntgen-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 10 31 −10 34 erg s −1 in a survey with an area of 10 deg 2 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.