rms-Flux Slope in MAXI J1820+070: A Measure of Disk-Corona Coupling

The linear rms-flux relation has been well established in different spectral states of all accreting systems. In this work, we study the evolution of the frequency-dependent rms-flux relation of MAXI J1820+070 during the initial decaying phase of its 2018 outburst with Insight-HXMT over a broad energy range of 1-150 keV. As the flux decreases, we first observe a linear rms-flux relation at frequencies from 2 mHz to 10 Hz, while such a relation breaks at varying times for different energies, leading to a substantial reduction in the slope. Moreover, we find that the low-frequency variability exhibits the highest sensitivity to the break, which occurs prior to the hard-to-hard state transition time determined through time-averaged spectroscopy, and the time deviation increases with energy. The overall evolution of the rms-flux slope and intercept suggests the presence of a two-component Comptonization system. One component is radially extended, explaining the strong disk-corona coupling before the break, while the other component extends vertically, contributing to a reduction of disk-corona coupling after the break. A further vertical expansion of the latter component is required to accommodate the dynamic evolution observed in the rms-flux slope. In conclusion, we suggest that the rms-flux slope in the 1-150 keV band can be employed as an indicator of disk-corona coupling, and the hard-to-hard state transition in MAXI J1820+070 could be partially driven by changes in the coronal geometry.