The bright black hole X-ray binary 4U 1543-47 during 2021 outburst. A clear state transition from super-Eddington to sub-Eddington accretion revealed by Insight-HXMT

We present a detailed analysis of the observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient 4U 1543-47 during its outburst in 2021. We find a clear state transition during the outburst decay of the source. Using previous measurements of the black hole mass and distance to the source, the source luminosity during this transition is close to the Eddington limit. The light curves before and after the transition can be fitted by two exponential functions with short (~16 d) and long (~130 d) decay time-scales, respectively. We detect strong reflection features in all observations that can be described with either the RELXILLNS or REFLIONX_BB reflection models, both of which have a black-body incident spectrum. In the super-Eddington state, we observe a Comptonized component characterized by a low electron temperature of approximately 2.0 keV. We suggest that this component appears exclusively within the inner radiation-pressure-dominated region of the supercritical disc as a part of the intrinsic spectrum of the accretion disc itself. This feature vanishes as the source transitions into the sub-Eddington state. The emissivity index of the accretion disc in the reflection component is significantly different before and after the transition, ~3.0-5.0 and ~7.0-9.0 in the super- and sub-Eddington states, respectively. Based on the reflection geometry of returning disc radiation, the geometrically thicker the accretion disc, the smaller the emissivity index. Therefore, we propose that the transition is primarily driven by the change of the accretion flow from a supercritical to a thin disc configuration.