Super-critical accretion of BeXRB SXP 15.3

Abstract

We have studied the temporal and spectral properties of SXP 15.3 observed by \textit{NuSTAR} in hard energy range 3-79 keV during late 2018. The timing analysis of \textit{NuSTAR} observation predicts coherent pulsation at $15.2388\;\pm\;0.0002\;s$. The pulse profiles in different energy bands demonstrates energy dependence. The shape of the pulse profile was generally suggestive of a fan-beamed dominated pattern, which when combined with the measured luminosity predicts that the source may be accreting in the super-critical regime. Non-monotonic increase of Pulse Fraction was observed with energy. The \textit{NuSTAR} observation finds that the pulse period of the source have spun-up at a rate of --0.0176 s $yr^{-1}$ when compared to the previous analysis by the same observatory more than 1 year ago. The source flux in the present \textit{NuSTAR} study in 3-79 keV energy range is $\sim\;1.36\;\times\;10^{-10}\;erg\;cm^{-2}\;s^{-1}$ which corresponds to a luminosity of $\sim\;6\;\times\;10^{37}\;erg\;s^{-1}$. The cyclotron line energy of the source is detected at $\sim$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 luminosity with time using 2017 and 2018 \textit{Swift/XRT} observations. The analysis of \textit{Swift/XRT} data reveals that the photon index are positively correlated with the source luminosity which is a characteristic of super-critical accretion phenomena.