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.Comment: Accepted for publication in Astronomy Letter

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

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

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

We report the discovery of the new long-period X-ray pulsar SRGA J204318.2+443815/SRGe J204319.0+443820 in a Be binary system. The source was found in the second all-sky survey by the Mikhail Pavlinsky ART-XC telescope on board the SRG mission. The follow-up observations with XMM-Newton, NICER, and NuSTAR allowed us to discover a strong coherent signal in the source light curve with a period of ~742 s. The pulsed fraction was found to depend on an increase in energy from ~20% in soft X-rays to >50% at high energies, as is typical for X-ray pulsars. The source has a quite hard spectrum with an exponential cutoff at high energies and a bolometric luminosity of LX ≃ 4 × 1035 ergs-1. The X-ray position of the source is found to be consistent with the optical transient ZTF18abjpmzf, located at a distance of ~8.0 kpc. Dedicated optical and infrared observations with the RTT-150, NOT, Keck, and Palomar telescopes revealed a number of emission lines (Ha, He I, and the Paschen and Braket series) with a strongly absorbed continuum. According to the SRG scans and archival XMM-Newton data, the source flux is moderately variable (by a factor of 4-10) on timescales of several months and years. All this suggests that SRGA J204318.2+443815/SRGe J204319.0+443820 is a new quasi-persistent low-luminosity X-ray pulsar in a distant binary system with a Be-star of the B0-B2e class. Thus the SRG observatory allowed us to unveil a hidden population of faint objects, including a population of slowly rotating X-ray pulsars in Be systems