{\it Insight}-HXMT View of the BHC Swift J1727.8-1613 during its outburst in 2023

Abstract

The transient Galactic black hole candidate Swift\,J1727.8-1613 went through an outburst for the very first time in August 2023 and lasted for almost 6 months. We study the timing and spectral properties of this source using publicly available archival {\it Insight}-HXMT data for the first 10 observation IDs that last from MJD 60181 to 60198 with a total of 92 exposures for all three energy bands. We have detected quasi-periodic oscillations (QPOs) in a frequency range of $0.21 \pm 0.01$ - $1.86 \pm 0.01$ Hz by fitting the power density spectrum. Based on the model-fitted parameters and properties of the QPOs, we classify them as type-C in nature. We also conclude that the origin of the QPOs could be the shock instabilities in the transonic advective accretion flows around black holes. The spectral analysis was performed using simultaneous data from the three on-board instruments LE, ME, and HE of \textit{Insight}-HXMT in the broad energy band of $2-150 $ keV. To achieve the best fit, spectral fitting required a combination of models e.g. interstellar absorption, power-law, multi-color disk-blackbody continuum, Gaussian emission/absorption, and reflection by neutral material. From the spectral properties, we found that the source was in an intermediate state at the start of the analysis period and was transitioning to the softer states. The inner edge of the accretion disk moved inward in progressive days following the spectral nature. We found that the source has a high inclination of $78^\circ-86^\circ$. The hydrogen column density from the model fitting varied in the range of $(0.12 \pm 0.02 - 0.39 \pm 0.08)\times10^{22}$ cm$^{-2}$.