Time-lag Between Disk and Corona Radiation Leads to Hysteresis Effect Observed in Black hole X-Ray Binary MAXI J1348-630

• Published in: Astrophysical journal. Letters Vol. 915; no. 1; p. L15
• Main Authors: Weng, Shan-Shan, Cai, Zhen-Yi, Zhang, Shuang-Nan, Zhang, Wei, Chen, Yu-Peng, Huang, Yue, Tao, Lian
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.07.2021; IOP Publishing
• Subjects: Accretion; Accretion disks; Active galactic nuclei; Black hole physics; Black holes; Brightening; Corona; Decay rate; Energy bands; Hysteresis; Low-mass x-ray binary stars; Luminosity; Outbursts; Radiation; Soft x rays; X ray binaries; X ray stars; X-ray astronomy; X-rays
• DOI: 10.3847/2041-8213/ac0a7b
• ISSN: 2041-8205

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 referred to is 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-called “q”-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 of 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 the “q”-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.