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Accretion is an essential physical process in black-hole X-ray binaries(BHXRBs) and active galactic nuclei. The properties of accretion flows andtheir radiation were originally considered to be uniquely determined by themass accretion rate of the disk; however, the ``hysteresis effect'' observedduring outbursts of nearly all BHXRBs seriously challenges this paradigm. Thehysteresis effect is referred to that the hard-to-soft state transition in thefast-rise stage occurs at much higher luminosity than the soft-to-hard statetransition in the slow-decay stage. That is, the same source can show differentspectral/temporal properties at the same luminosity. Phenomenologically, thiseffect is also represented as the so-called ``q''-shaped hardness-intensitydiagram, which has been proposed as a unified scene for BHXRBs. However, thereis still a lack of quantitative theoretical interpretation and observationalunderstanding on the ``q''-diagram. Here, we present a detailed time-laganalysis of a recently found BHXRB, MAXI J1348-630, intensively monitored byInsight-HXMT over a broad energy band (1-150 keV). We find the firstobservational evidence that the observed time-lag between radiations of theaccretion disk and the corona leads naturally to the hysteresis effect and the``q''-diagram. Moreover, complemented by the quasi-simultaneous Swift data, weachieve a panorama of the accretion flow: the hard X-ray outburst from thecorona heats and subsequently induces the optical brightening in the outer diskwith nearly no lag; thereafter, the enhanced accretion in the outer diskpropagates inward, generating the delayed soft X-ray outburst at the viscoustimescale of ~ 8-12 days.

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