Spectral Analysis of the X-Ray Flares in the 2023 Outburst of the New Black Binary Transient Swift J1727.8–1613 Observed with Insight-HXMT

Abstract The new black hole transient Swift J1727.8–1613 exhibited a series of X-ray flares during its 2023 outburst extensively observed with Insight-HXMT. We analyze the spectra of the flaring period using a series of models consisting of a multicolor disk and several different nonthermal components, and several consistent conclusions are obtained among these models. First, Swift J1727.8–1613 was in the transition process from the hard intermediate state to the very high state (VHS) during the first flaring period (MJD 60197–60204), and afterward, it exhibited typical VHS parameter characteristics, such as the high temperature of the disk inner radius and a steep power-law spectrum with a photon index of 2.6. Second, the flares in the VHS are characterized by a rapid increase in the flux of the accretion disk, accompanied by a simultaneous rapid expansion of the inner radius, which could be apparent if the accretion disk hardening factor varies significantly. The strong power-law component during the VHS is likely produced by the synchrotron self-Compton process in the relativistic jets, which is in agreement with the observed weak reflection component and lack of correlation with the disk component.