arXiv subjects

A. Semena

Publications and source records attributed to A. Semena.

13 recordsLinked to original sources

arXiv2025

Inflight calibration of SRG/ART-XC point spread function at large off-axis angles

The knowledge of the point spread function (PSF) of the Mikhail Pavlinsky Astronomical Roentgen Telescope - X-ray Concentrator (ART-XC) telescope aboard the Spectrum-Roentgen-Gamma (SRG) observatory plays an especially crucial role in the detection of point X-ray sources in the all-sky survey and the studies of extended X-ray objects with low surface brightness. In this work, we calibrate the far off-axis shape of the ART-XC PSF using in-flight data of Sco X-1 and the Crab Nebula, in all-sky survey or scan mode, respectively. We demonstrate that the so-called "slewing" ART-XC PSF (in contrast to the on-axis PSF), in convolution with the detector pixels, is consistent with ground calibration performed at the Marshall Space Flight Center, and can be used to model the PSF up to large off-axis distances in all-sky survey or scan modes. The radial profile of the Crab Nebula in the 4-12 keV band shows an extended structure out to ~150" and is consistent with Sco X-1 at larger off-axis angles. Finally, we performed an analytic parametrization of the slewing ART-XC PSF as a function of energy.

astro-ph.IM↗

arXiv2024

SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first five surveys

We present an updated catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum-Roentgen-Gamma (SRG) observatory during its all-sky survey. It is based on the data of the first four and the partially completed fifth scans of the sky (ARTSS1-5). The catalog comprises 1545 sources detected in the 4-12 keV energy band. The achieved sensitivity ranges between $\sim 4\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ near the ecliptic plane and $\sim 7\times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$ near the ecliptic poles, which is a $\sim$30-50% improvement over the previous version of the catalog based on the first two all-sky scans (ARTSS12). There are $\sim 130$ objects, excluding the expected contribution of spurious detections, that were not known as X-ray sources before the SRG/ART-XC all-sky survey. We provide information, partly based on our ongoing follow-up optical spectroscopy program, on the identification and classification of the majority of the ARTSS1-5 sources (1463), of which 173 are tentative at the moment. The majority of the classified objects (964) are extragalactic, a small fraction (30) are located in the Local Group of galaxies, and 469 are Galactic. The dominant classes of objects in the catalog are active galactic nuclei (911) and cataclysmic variables (192).

astro-ph.HE↗

arXiv2024

SRG/ART-XC Galactic Bulge deep survey. I. Maximum likelihood source detection algorithm for X-ray surveys

We describe an X-ray source detection method entirely based on the maximum likelihood analysis, in application to observations with the ART-XC telescope onboard the Spectrum Roentgen Gamma observatory. The method optimally combines the data taken at different conditions, a situation commonly found in scanning surveys or mosaic observations with a telescope with a significant off-axis PSF distortion. The method can be naturally extended to include additional information from the X-ray photon energies, detector grades, etc. The likelihood-based source detection naturally results in a stable and uniform definition of detection thresholds under different observing conditions (PSF, background level). This greatly simplifies the statistical calibration of the survey needed to, e.g., obtain the $\log N - \log S$ distribution of detected sources or their luminosity function. The method can be applied to the data from any imaging X-ray telescope.

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arXiv2023

Hard X-rays and QPO in Swift J1727.8-1613: the rise and plateau of the 2023 outburst

We report on the detection of type-C quasi-periodic oscillations during the initial stages of the outburst of Swift J1727.8-1613 in 2023. Using data of the INTEGRAL observatory along with the data of the SRG/ART-XC and Swift/XRT telescopes the fast growth of the QPO frequency was traced. We present a hard X-ray lightcurve that covers the initial stages of the 2023 outburst - the fast rise and plateau - and demonstrate that the QPO frequency was stable during the plateau. The switching from type-C to type-B QPO was detected with the beginning of the source flaring activity. We have constructed a broad-band spectrum of Swift J1727.8-1613 and found an additional hard power-law spectral component extending at least up to 400 keV. Finally, we have obtained an upper limit on the hard X-ray flux at the beginning of the optical outburst and estimated the delay of the X-ray outburst with respect to the optical one.

astro-ph.HE↗

arXiv2023

Simultaneous and panchromatic observations of the Fast Radio Burst FRB 20180916B

Aims. Fast Radio Bursts are bright radio transients whose origin has not yet explained. The search for a multi-wavelength counterpart of those events can put a tight constrain on the emission mechanism and the progenitor source. Methods. We conducted a multi-wavelength observational campaign on FRB 20180916B between October 2020 and August 2021 during eight activity cycles of the source. Observations were led in the radio band by the SRT both at 336 MHz and 1547 MHz and the uGMRT at 400 MHz. Simultaneous observations have been conducted by the optical telescopes Asiago (Galileo and Copernico), CMO SAI MSU, CAHA 2.2m, RTT-150 and TNG, and X/Gamma-ray detectors on board the AGILE, Insight-HXMT, INTEGRAL and Swift satellites. Results. We present the detection of 14 new bursts detected with the SRT at 336 MHz and seven new bursts with the uGMRT from this source. We provide the deepest prompt upper limits in the optical band fro FRB 20180916B to date. In fact, the TNG/SiFAP2 observation simultaneous to a burst detection by uGMRT gives an upper limit E_optical / E_radio < 1.3 x 10^2. Another burst detected by the SRT at 336 MHz was also co-observed by Insight-HMXT. The non-detection in the X-rays yields an upper limit (1-30 keV band) of E_X-ray / E_radio in the range of (0.9-1.3) x 10^7, depending on which model is considered for the X-ray emission.

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arXiv2021

First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z<0.6

We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0<l<180 deg hemisphere by eROSITA during its second sky survey (10 June - 14 December 2020) and confirmed by optical follow-up observations. The most distant event occurred at z=0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion onto black holes of a few 10^3 to 10^8 M_Sun, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other 9 TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with Lopt/Lx<0.3 (Lopt and Lx being the g-band and 0.2-6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of -0.6+/-0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1+/-0.5)10^-5 TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disk formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of ~2, which should allow a detection of ~700 TDEs by the end of the SRG survey.

astro-ph.HE↗

arXiv2021

SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first year

We present a first catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory in the 4-12 keV energy band during its ongoing all-sky survey. The catalog comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (December 2019 - December 2020). The achieved sensitivity to point sources varies between ~4x10^-12 erg/s/cm2 near the ecliptic plane and ~8x10^-13 erg/s/cm2 (4-12 keV) near the ecliptic poles, and the typical localization accuracy is ~15 arcsec. Of the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei, AGN; and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables, CVs; and low- and high-mass X-ray binaries). For 114 sources, ART-XC has detected X-rays for the first time. Although the majority of these (~80) are expected to be spurious (given the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of four years, we can expect the ART-XC survey in the 4-12 keV band to significantly surpass previous surveys that were carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.

astro-ph.HE↗

arXiv2021

Identification of 3 cataclysmic variables detected by the ART-XC and eROSITA telescopes aboard SRG during the all-sky X-ray survey

We report the discovery of three previously unknown cataclysmic variables in the data of the first year of the all-sky X-ray survey by the SRG orbital observatory. The sources were selected due to their brightness in the 4--12 keV band in the data of the Mikhail Pavlinsky ART-XC telescope. They are also detected by the eROSITA telescope, which provided accurate localizations and spectral data for broad-band spectral analysis. All three objects had been previously known as X-ray sources from the ROSAT all-sky survey and XMM-Newton slew survey, but their nature remained unknown. The X-ray spectra obtained by eROSITA and ART-XC are consistent with optically thin thermal emission with a temperature kT>~15 keV for SRGAJ194638.9+704552 and SRGAJ225412.8+690658 and kT>~5 keV for SRGAJ204547.8+672642. This, together with the inferred high X-ray luminosities ($2\times 10^{32}$-$3\times 10^{33}$ erg s$^{-1}$), strongly suggests that all three sources are CVs. We have obtained optical photometry and spectroscopy for these objects using the AZT-33IK 1.6-m telescope of the Sayan Observatory. The optical properties confirm the CV nature of the objects. We conclude that SRGAJ194638.9+704552 is an intermediate polar, SRGAJ204547.8+672642 is most likely a polar or an intermediate polar, and SRGAJ225412.8+690658 can be either a magnetic or a non-magnetic CV. We also measured an orbital period of 2.98~hours for SRGAJ204547.8+672642, based on TESS data. Three out of the planned eight SRG all-sky surveys have now been completed. We expect to find plenty of new CVs during the survey and to continue our optical follow-up program.

astro-ph.HE↗

arXiv2021

Wide-field X-ray observations of the supernova remnant Puppis A with the SRG/ART-XC telescope

The Spectrum-Roentgen-Gamma (SRG) observatory is currently conducting its 4-year all-sky X-ray survey, started on December 12, 2019. The survey is periodically interrupted for technological operations with the spacecraft. These time intervals are usually used by the Mikhail Pavlinsky ART-XC telescope to perform calibrations. In this context, SRG carried out scanning observations of the Puppis A supernova remnant (SNR) with the aim to check the imaging performance of ART-XC and to optimize the technique of image reconstruction for extended objects. Using the unique imaging capabilities of ART-XC and its uniform coverage of the entire Puppis A region, we attempted to investigate the morphology of this SNR at energies >4 keV, and to search for previously unknown X-ray sources. Puppis A was observed in 2019-2020, conducting 1.5 x 1.5 degrees shallow surveys with an exposure of 36 hours. Additional deep pointed observations of the central part of Puppis A were carried out in 2021 lasted 31 hours to highlight the morphology of the extended emission. The X-ray emission of the Puppis A was significantly detected as an extended structure in the 4-6 keV energy band. The morphology of the emission is in general agreement with that observed in soft X-rays previously. The deep sky image of Puppis A obtained with the ART-XC telescope is characterized by a typical SNR shell rim morphology, an extended emission and a bright emission knot in the north-eastern part of the supernova shell. Also, four point X-ray sources have been detected, including three objects identified in catalogs, and one newly discovered X-ray emitter.

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arXiv2021

SRGA J124404.1-632232/SRGU J124403.8-632231: a new X-ray pulsar discovered in the all-sky survey by SRG

Ongoing all-sky surveys by the the eROSITA and the Mikhail Pavlinsky ART-XC telescopes on-board the Spectrum Roentgen Gamma (SRG) mission have already revealed over a million of X-ray sources. One of them, SRGA J124404.1-632232/SRGU J124403.8-632231, was detected as a new source in the third (of the planned eight) consecutive X-ray surveys by ART-XC. Based on the properties of the identified optical counterpart it was classified as a candidate X-ray binary (XRB). We report on the follow-up observations of this source with Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels Swift Observatory (Swift), and the Southern African Large Telescope (SALT), which allowed us to unambiguously confirm the initial identification and establish SRGU J124403.8-632231 as a new X-ray pulsar with a spin period of ~538 s and a Be-star companion, making it one of the first Galactic X-ray pulsars discovered by SRG.

astro-ph.HE↗

arXiv2021

Discovery of the 5 keV cyclotron line followed by three harmonics in SwiftJ1626.6-5156

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.

astro-ph.HE↗

arXiv2021

The ART-XC telescope on board the SRG observatory

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.

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arXiv2016

Sample of Cataclysmic Variables from 400d X-ray Survey

We present a sample of eight cataclysmic variables (CVs) identified among the X-ray sources of the 400 square degree (400d) X-ray ROSAT/PSPC survey. Based on this sample, we have obtained preliminary constraints on the X-ray luminosity function of CVs in the solar neighbourhood in the range of low luminosities, L_X=~1e29-1e30 erg/s (0.5-2 keV). We show that the logarithmic slope of the CV luminosity function in this luminosity range is less steep than that at L_X>1e31 erg/s. Our results show that of order of thousand CVs will be detected in the SRG/eROSITA all-sky survey at high Galactic latitudes, which will allow to obtain much more accurate measurements of their X-ray luminosity function.

astro-ph.HE↗