Time-lag between disk and corona radiation leads to hysteresis effect observed in black-hole X-ray binary MAXI J1348-630

Abstract: Accretion is an essential physical process in black-hole X-ray binaries (BHXRBs) and active galactic nuclei. The properties of accretion flows and their radiation were originally considered to be uniquely determined by the mass accretion rate of the disk; however, the hysteresis effect'' observed during outbursts of nearly all BHXRBs seriously challenges this paradigm. The hysteresis effect is referred to that the hard-to-soft state transition in the fast-rise stage occurs at much higher luminosity than the soft-to-hard state transition in the slow-decay stage. That is, the same source can show different spectral/temporal properties at the same luminosity. Phenomenologically, this effect is also represented as the so-calledq''-shaped hardness-intensity diagram, which has been proposed as a unified scene for BHXRBs. However, there is still a lack of quantitative theoretical interpretation and observational understanding on the q''-diagram. Here, we present a detailed time-lag analysis of a recently found BHXRB, MAXI J1348-630, intensively monitored by Insight-HXMT over a broad energy band (1-150 keV). We find the first observational evidence that the observed time-lag between radiations of the accretion disk and the corona leads naturally to the hysteresis effect and theq''-diagram. Moreover, complemented by the quasi-simultaneous Swift data, we achieve a panorama of the accretion flow: the hard X-ray outburst from the corona heats and subsequently induces the optical brightening in the outer disk with nearly no lag; thereafter, the enhanced accretion in the outer disk propagates inward, generating the delayed soft X-ray outburst at the viscous timescale of ~ 8-12 days.