Supercritical accretion of BeXRB SXP 15.3

Abstract

We have studied the temporal and spectral properties of SXP 15.3, observed by NuSTAR in the hard energy range 3–79 keV during late 2018. Timing analysis of the NuSTAR observations predicts coherent pulsation at $15.2388\,\,\pm \,\,0.0002\, \mathrm{s}$. The pulse profiles in different energy bands demonstrate energy dependence. The shape of the pulse profile was generally suggestive of a fan-beam-dominated pattern, which, when combined with the measured luminosity, predicts that the source may be accreting in the supercritical regime. A non-monotonic increase in pulse fraction with energy was observed. The NuSTAR observations show that the pulse period of the source has spun up at a rate of −0.0176 s yr−1 compared with the previous analysis by the same observatory more than 1 year ago. The source flux in the present NuSTAR study in the 3–79 keV energy range is ${\sim }1.36\,\,\times \,\,10^{-10}\, \mathrm{erg\, cm}^{-2}\, \mathrm{s}^{-1}$, which corresponds to a luminosity of ${\sim }6\,\,\times \,\,10^{37}\, \mathrm{erg\, s}^{-1}$. Cyclotron line energy of the source is detected at ∼5 keV. Pulse-phase-resolved spectroscopy shows that the cyclotron line energy varies significantly with pulse phase and the photon index becomes softer with increasing flux. In addition, we have studied the evolution of the luminosity with time using 2017 and 2018 Swift/XRT observations. Analysis of the Swift/XRT data reveals that the photon index is positively correlated with the source luminosity, which is a characteristic of supercritical accretion phenomena.