2025ApJ...983...23C

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.