A possible overall scenario for the outburst evolution of MAXI J1820+070 revealed by Insight-HXMT

TL;DR

This study analyzes the spectral and timing evolution of MAXI J1820+070 during its 2018 outburst, revealing energy-dependent lags, changes in the corona, and a revised outburst hysteresis shape, providing insights into black hole accretion processes.

Contribution

It presents a detailed analysis of energy-dependent lags and corona evolution during the outburst, offering a new perspective on the outburst hysteresis shape in black hole transients.

Findings

Low-energy photons lag high-energy photons in the rise, opposite in decay.

The high-frequency break varies more in the hard X-ray band.

The corona coverage fraction changes significantly during the outburst.

Abstract

We study the spectral and temporal properties of the black hole X-ray transient binary MAXI J1820+070 during the 2018 outburst with Insight-HXMT observations. The outburst of MAXI J1820+070 can be divided into three intervals. For the two intervals of the outburst, we find that low-energy (below 140 keV) photos lag high-energy (140-170 keV) ones, while in the decay of the outburst, high-energy photons lag low-energy photons, both with a time scale of the order of days. Based on these results, the canonical hysteresis effect of the 'q' shape in the hardness-intensity diagram can be reformed into a roughly linear shape by taking into account the lag corrections between different energy bands. Time analysis shows that the high-frequency break of hard X-rays, derived from the power density spectrum of the first interval of the outburst is, in general, larger and more variable than that of soft X-rays. The spectral fitting shows that the coverage fraction of the hard X-rays drops sharply at the beginning of the outburst to around 0.5, then increases slightly. The coverage fraction drops to roughly zero once the source steps into a soft state and increases gradually to unity when the source returns to a low hard state. We discuss the possible overall evolution scenario of corona hinted from these discoveries.