arXiv subjects

A. N. Semena

Publications and source records attributed to A. N. Semena.

17 recordsLinked to original sources

arXiv2025

SRG/ART-XC discovery of pulsations from RX J0535.0-6700: another X-ray pulsar in the LMC

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.

astro-ph.HE↗

arXiv2024

SRG/ART-XC Galactic Plane Survey near Galactic Longitude $l\simeq20^\circ$: Catalog of Sources

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 $l\simeq20$ deg (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 $\simeq24$ sq. deg with a median sensitivity of $8\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ (at 50% detection completeness) in the 4$-$12 keV. As a result, we have detected 29 X-ray sources at a statistically significant level, 11 of which have not been detected previously by other X-ray observatories. Preliminary estimates show that four of them can presumably be extragalactic in nature. We also show that the source SRGA J183220.1$-$103508 (CXOGSG J183220.8$-$103510), is most likely a galaxy cluster containing a bright radio galaxy at redshift $z\simeq0.121$.

astro-ph.HE↗

arXiv2024

SRG/ART-XC discovery of SRGAJ144459.2-604207: a well-tempered bursting accreting millisecond X-ray pulsar

We report on the discovery of the new accreting millisecond X-ray pulsar SRGAJ144459.2-604207 using the SRG/ART-XC data. The source was observed twice in February 2024 during the declining phase of the outburst. Timing analysis revealed a coherent signal near 447.8~Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with the circular orbit with a period of $\sim5.2$~h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can well be modelled by emission from two circular spots partially eclipsed by the accretion disk. Additionally, during our 133~ks exposure observations, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and do not show any signs of photospheric radius expansion. The burst rate decreases linearly from one per $\sim$1.6~h at the beginning of observations to one per $\sim$2.2~h at the end and anticorrelates with the persistent flux. Spectral evolution during the bursts is consistent with the models of the neutron star atmospheres heated by accretion and imply a neutron star radius of 11--12~km and the distance to the source of 8--9~kpc. We also detected 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.

astro-ph.HE↗

arXiv2024

All-sky limits on Sterile Neutrino Galactic Dark Matter obtained with SRG/ART-XC after two years of operations

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.

astro-ph.HE↗

arXiv2023

X-ray emission from Westerlund 2 detected by SRG/ART-XC and Chandra: search for radiation of TeV leptons

We present the results of current observations of the young compact cluster of massive stars Westerlund 2 with the Mikhail Pavlinsky ART-XC telescope aboard the Spectrum-Roentgen-Gamma (SRG) observatory which we analysed together with the archival Chandra data. In general, Westerlund 2 was detected over the whole electromagnetic spectrum including high-energy gamma rays, which revealed a cosmic ray acceleration in this object to the energies up to tens of TeV. The detection of Westerlund 2 with ART-XC allowed us to perform a joint spectral analysis together with the high resolution Chandra observations of the diffuse emission from a few selected regions in the vicinity of the Westerlund 2 core in the 0.4 - 20 keV range. To fit the Westerlund 2 X-ray spectrum above a few keV one needs either a non-thermal power-law emission component, or a hot plasma with temperatures $\sim$ 5 keV. Our magnetohydrodynamic modeling of the plasma flows in Westerlund 2 shows substantially lower electron temperatures in the system and thus the presence of the non-thermal component is certainly preferable. A kinetic model of the particle acceleration demonstrated that the non-thermal component may originate from the synchrotron radiation of multi-TeV electrons and positrons produced in Westerlund 2 in accordance with the TeV photons detection from the source.

astro-ph.HE↗

arXiv2023

X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

X Persei is a persistent low-luminosity X-ray pulsar of period of $\sim$835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above $10^{13}$ G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to $\sim$20 per cent over the pulse with a positive correlation with the pulsed X-ray flux. The polarization angle shows significant variation and makes two complete revolutions during the pulse period resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008$-$57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered with its value increasing from 0% at $\sim$2 keV to 30% at 8 keV.

astro-ph.HE↗

arXiv2023

Study of the X-ray Pulsar IGR J21343+4738 based on NuSTAR, Swift, and SRG data

We present the results of our study of the X-ray pulsar IGR J21343+4738 based on NuSTAR, Swift, and SRG observations in the wide energy range 0.3 - 79 keV. The absence of absorption features in the energy spectra of the source, both averaged and phase-resolved ones, has allowed us to estimate the upper and lower limits on the magnetic field of the neutron star in the binary system, $B<2.5\times10^{11}$G and $B>3.4 \times 10^{12}$G, respectively. The spectral and timing analyses have shown that IGR J21343+4738 has all properties of a quasi-persistent X-ray pulsar with a pulsation period of $322.71\pm{0.04}$s and a luminosity $L_{x} \simeq3.3$ $\times10^{35}$erg s$^{-1}$. The analysis of the long-term variability of the object in X-rays has confirmed the possible orbital period of the binary system $\sim 34.3$ days previously detected in the optical range.

astro-ph.HE↗

arXiv2022

Search for pre-burst emission from binary neutron star mergers with Spectrum-Roentgen-Gamma

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.

astro-ph.HE↗

arXiv2021

Peculiar X-ray transient SRGA J043520.9+552226/AT2019wey discovered with SRG/ART-XC

Context: During the ongoing all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory should discover new X-ray sources, many of which can be transient. Here we report on the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226=SRGe J043523.3+552234 - the high-energy counterpart of the optical transient AT2019wey. Aims: Thanks to its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using a synergy with current public optical surveys, we are aiming at revealing the nature of these transients to study its parent populations. The SRGA J043520.9+552226 is the first transient detected by ART-XC which has a bright optical counterpart suitable for further studies. Methods: We have used available public X-ray and optical data and observations with SRG, INTEGRAL, NuSTAR, NICER and ground-based telescopes to investigate the source spectral energy distributions at 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 CMO SAI MSU telescopes, we propose the source to be a black hole in a low-mass close X-ray binary system.

astro-ph.HE↗

arXiv2021

SRG/ART-XC discovery of SRGA J204318.2+443815: towards the complete population of faint X-ray pulsars

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$_α$, 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.

astro-ph.HE↗

arXiv2021

Losing a minute every two years: SRG X-ray view on the rapidly accelerating X-ray pulsar SXP1323

SXP 1323 is a peculiar high-mass X-ray binary located in the Small Magellanic Cloud, renowned for its rapid spin-up. We investigate for the first time broadband X-ray properties of SXP1323, as observed by the Mikhail Pavlinsky ART-XC and eRosita telescopes on board the SRG observatory. Using ART-XC and eRosita, data we produced first broadband 1-20 keV X-ray spectrum and estimated pulsed fraction above 8 keV. With the addition of archival XMM-Newton observations we traced evolution of the SXP 1323 spin period over the last five years and found that after 2016 the source switched to a linear spin-up with rate of -29.9 s yr$^{-1}$. Broadband X-ray spectrum is typical for accreting X-ray pulsars, with steep powerlaw index ($Γ$=-0.15) and exponential cutoff energy of 5.1 keV. No significant difference between spectra obtained in states with and without pulsations were found.

astro-ph.HE↗

arXiv2021

Phase-Resolved Spectroscopy of the Magnetar SGR J1745-2900 Based on Data from the NuSTAR Observatory

The magnetar SGR J1745-2900 located in the vicinity of the supermassive black hole Sgr A$^{\star}$ was detected during its X-ray outburst with the Swifht/XRT telescope in April 2013. For several months after its detection the source was observed with the NuSTAR observatory, which allowed pulsations with a period $\sim3.76$ s to be recorded. Using these observations, we have studied in detail the dependence of the pulse profile and the pulsed fraction on the energy and intensity of the magnetar. The pulsed fraction in the 3-5 and 5-10 keV energy bands is shown to be 40-50%, slightly increasing with decreasing flux. We have performed phase-resolved spectroscopy for the source in the energy band from 3 to $\sim$40 keV and show that the temperature of the emitting regions remains fairly stable during the pulse, while their apparent size changes significantly with phase.

astro-ph.HE↗

arXiv2019

Features of the Accretion in the EX Hydrae System: Results of Numerical Simulation

A two dimensional numerical model in the axisymmetric approximation that describes the flow structure in the magnetosphere of the white dwarf in the EX Hya system has been developed. Results of simulations show that the accretion in EX Hya proceeds via accretion columns, that are not closed and have curtain-like shapes. The thickness of the accretion curtains depends only weakly on the thickness of the accretion disk. This thickness developed in the simulations does not agree with observations. It is concluded that the main reason for the formation of thick accretion curtains in the used model is the assumption that the magnetic field penetrates fully into the plasma of the disk. An analysis based on simple estimates shows that a diamagnetic disk that fully or partially shields the magnetic field of the star may be a more attractive explanation for the observed features of the accretion in EX Hya.

astro-ph.SR↗

arXiv2018

Low-Frequency Quasi-Periodic Oscillations in the X-ray Nova MAXI J1535-571 at the Initial Stage of Its 2017 Outburst

We report the discovery of low-frequency quasi-periodic oscillations (QPOs) in the power spectrum of the X-ray nova MAXI J1535-571 at the initial stage of its outburst in September 2017. Based on data from the SWIFT and INTEGRAL instruments, we have traced the evolution of the QPO parameters (primarily their frequency) with time and their correlation with changes in the X-ray spectrum of the source (changes in the emission flux and hardness). We place constraints on the theoretical QPO generation models.

astro-ph.HE↗

arXiv2017

Modeling of High-Frequency Variability in X-ray Binaries with Black Holes

The properties of the aperiodic variability in X-ray binaries with black holes are considered. The power spectra of the luminosity variability for a flat accretion disk that is an emission source with a power-law energy spectrum have been modeled. At low frequencies the derived power spectrum has the form of a power law with a slope $ρ\approx-1$ and a cutoff at a frequency approximately equal to the fluctuations characteristic frequency at the disk inner edge; at higher frequencies the power spectrum has a complex form. The high-frequency variability is suppressed due to the arrival time delays of the photons emerging in the different parts of the disk. The presence of azimuthal accretion rate fluctuations in the disk and the azimuthal non-uniformity of the disk surface brightness in the observer's imaginary plane caused by the relativistic effects give rise to an additional variability at frequencies $\sim$200 Hz.

astro-ph.HE↗

arXiv2017

Sample of Cataclysmic Variables from 400d X-ray Survey

We present a sample of cataclysmic variables (CVs) identified among the X-ray sources from the 400 square degree X-ray survey based on ROSAT pointing data (400d). The procedure of the CV selection among the X-ray sources using additional optical and infrared data from Sloan Digital Sky Survey and WISE survey is described. The results of 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) are presented. Some observations have also been performed with the Sayan Observatory 1.6-m AZT-33IK telescope. Currently we selected eight CVs, four of which were found for the first time in our work. Based on this sample, we have obtained preliminary constraints on the CV X-ray luminosity function in the solar neighborhood in the low luminosity range, L_X ~ 10^29-10^30 erg s^-1 (0.5-2 keV). We show that the logarithmic slope of the CV X-ray luminosity function in this luminosity range is less steep than at L_X > 10^31 erg s^-1. From our CV X-ray luminosity function estimates it follows that few thousand CVs will be detected in theSpectrum-Roentgen-Gamma (SRG) observatory all-sky X-ray survey at high Galactic latitudes, which will allow to obtain much more accurate measurements of CV X-ray luminosity function in the luminosity range L_X < 10^30-10^31 erg s^-1.

astro-ph.HE↗

arXiv2016

Additional spectroscopic redshift measurements for galaxy clusters from the First Planck Catalogue

We present the results of spectroscopic redshift measurements for the galaxy clusters from the first all-sky Planck catalogue of the Sunyaev-Zeldovich sources, that have been mostly identified by means of the optical observations performed previously by our team (Planck Collaboration, 2015a). The data on 13 galaxy clusters at redshifts from z=~0.2 to z=~0.8, including the improved identification and redshift measurement for the cluster PSZ1 G141.73+14.22 at z=0.828, are provided. The measurements were done using the data from Russian-Turkish 1.5-m telescope (RTT-150), 2.2-m Calar Alto Observatory telescope, and 6-m SAO RAS telescope (Bolshoy Teleskop Azimutalnyi, BTA).

astro-ph.CO↗