The accretion flow geometry of MAXI J1820+070 through broadband noise research with Insight-HXMT

TL;DR

This study analyzes the broadband noise in MAXI J1820+070's X-ray emission, revealing energy-dependent variability and suggesting a complex accretion flow geometry with a truncated disk and multiple Comptonization regions.

Contribution

It provides a detailed broadband noise analysis with Lorentzian modeling, revealing energy-dependent properties and proposing a new accretion flow geometry model.

Findings

Broadband noise shows two main humps linked to different regions.

Characteristic frequencies increase sharply above 20-30 keV.

The accretion flow likely consists of a truncated disk and two Comptonization regions.

Abstract

Here we present a detailed study of the broadband noise in the power density spectra of the black hole X-ray binary MAXI J1820+070 during the hard state of its 2018 outburst, using the Hard X-ray Modulation Telescope (Insight-HXMT) observations. The broadband noise shows two main humps, which might separately correspond to variability from a variable disk and two Comptonization regions. We fitted the two humps with multiple Lorentzian functions and studied the energy-dependent properties of each component up to 100--150 keV and their evolution with spectral changes. The lowest frequency component is considered as the sub-harmonic of QPO component and shows different energy dependence compared with other broadband noise components. We found that although the fractional rms of all the broadband noise components mainly decrease with energy, their rms spectra are different in shape. Above $\sim$ 20--30 keV, the characteristic frequencies of these components increase sharply with energy, meaning that the high-energy component is more variable on short timescales. Our results suggest that the hot inner flow in MAXI J1820+070 is likely to be inhomogeneous. We propose a geometry with a truncated accretion disk, two Comptonization regions.