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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. [ABSTRACT FROM AUTHOR]

We present the results of our study of the transient X-ray pulsar GRO J2058+42 in a wide energy range in a state with a luminosity erg s . We have found that the pulse profile of the source, along with the pulse fraction, changed significantly in comparison with the previous NuSTAR observations performed when the pulsar was brighter approximately by a factor of 10. The position of the cyclotron line at keV in a narrow phase interval is consistent with the observations in the high state. Spectral analysis has shown that at high luminosities erg s the spectrum has a shape typical of accreting pulsars, whereas a two-component model should be used to describe the spectrum when the luminosity drops approximately by an order of magnitude, to erg s . This behavior fits into the model in which the low-energy part of the spectrum is formed in a hot spot, while the high-energy one is formed as a result of resonant Compton scattering by the infalling matter in the accretion channel above the neutron star surface. [ABSTRACT FROM AUTHOR]

We present the results of our spectral and timing analysis of the emission from the transient X-ray pulsar XTE J1946+274 based on the simultaneous NuSTAR and Swift/XRT observations in the broad energy range 0.3–79 keV carried out in June 2018 during a bright outburst. Our spectral analysis has confirmed the presence of a cyclotron absorption line at an energy keV in both averaged and phase-resolved spectra of the source. Phase-resolved spectroscopy has also allowed the variation in spectral parameters with neutron star rotation phase, whose period is s, to be studied. The energy of the cyclotron line is shown to change significantly (from to keV) on the scale of a pulse, with the line width and optical depth also exhibiting variability. The observed behavior of the cyclotron line parameters can be interpreted in terms of the model of the reflection of emission from a small accretion column (the source's luminosity at the time of its observations was erg s ) off the neutron star surface. The equivalent width of the iron line has been found to also change significantly with pulse phase. The time delay between the pulse and equivalent width profiles can be explained by the reflection of neutron star emission from the outer accretion disk regions. [ABSTRACT FROM AUTHOR]

We report results of a spectral and timing analysis of the X-ray pulsar XTE J1829−098 using data obtained with the NuSTAR observatory during an outburst in 2018 August. A strong absorption feature was detected at the energy of E cyc ≃ 15 keV in the source spectrum. This feature was interpreted as a cyclotron resonance scattering line corresponding to the magnetic field strength of the neutron star surface B ≃ 1.7 × 1012 G. The pulse phase-resolved spectroscopy shows that the cyclotron line is significantly detected at all phases of the pulse and its energy and other parameters are variable over the pulse period. The timing analysis of the source emission revealed strong pulsations with a period of P = 7.84480(2) s. The pulsed fraction is changed with the energy, including its local increase in the vicinity of the cyclotron line. Using the archival data of the RXTE observatory the presence of the cyclotron line in the spectrum of XTE J1829−098 was independently confirmed and tentative hint for the possible anticorrelation of the line energy with the source flux was revealed. [ABSTRACT FROM AUTHOR]