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arXiv · 2401.01515

RMS-flux slope in MAXI J1820+070: a measure of the disk-corona coupling

Yanan Wang·Shuang-Nan Zhang
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Abstract

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 the 2018 outburst with Insight-HXMT over a broad energy range 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 the reduction of the 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 1-150 keV band can be employed as an indicator of the disk-corona coupling and the hard-to-hard state transition in MAXI J1820+070 could be partially driven by the changes in the corona geometry.

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Yanan Wang, Shuang-Nan Zhang. 2024-01-03. RMS-flux slope in MAXI J1820+070: a measure of the disk-corona coupling. https://arxiv.org/abs/2401.01515

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