AstroSat Observations of GRO J2058+42 during the 2019 Outburst

Abstract

Abstract We present results from AstroSat observations of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed by the LAXPC and SXT instruments on AstroSat on 2019 April 10 during the declining phase of its latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 ± 0.0016 s and a spin-up rate of (1.65 ± 0.06) × 10−11 Hz s−1. Intermittent flaring was detected in light curves between the 3 and 80 keV energy bands, with an increase in intensity of up to 1.8 times its average intensity. Pulse profiles obtained between the 3 and 80 keV energy bands of the pulsar showed strong dependence on energy. During AstroSat observations, a broad peak was consistently observed in the power density spectrum of the source with a peak oscillation frequency of 0.090 Hz along with its higher harmonics, which may be due to quasi-periodic oscillations, a commonly observed phenomenon in transient X-ray pulsars during their outburst. AstroSat observations also detected cyclotron absorption features in its spectrum corresponding to (9.7–14.4) keV, (19.3–23.8) keV, and (37.8–43.1) keV. The pulse-phase-resolved spectroscopy of the source showed a phase-dependent variation in its energy and the relative strength of these features. The spectrum was well fitted with an absorbed blackbody, Fermi–Dirac cutoff model and alternatively with an absorbed CompTT model. Both of these models were combined with an Fe line and three Gaussian absorption lines to account for the observed cyclotron resonance scattering features in the spectrum.