Accretion Geometry in the Hard State of the Black-Hole X-Ray Binary MAXI J1820+070

TL;DR

This study analyzes X-ray spectra from the black-hole binary MAXI J1820+070, revealing a structured accretion flow with variable inner disk radius and distinct spectral components, shedding light on accretion geometry during the hard state.

Contribution

It provides new insights into the accretion disk structure and geometry, demonstrating the presence of multiple spectral components and their evolution during the hard state.

Findings

Inner disk radius decreases from >100 to ~10 gravitational radii during the hard state.

The accretion flow is structured with at least two spectral components.

Harder component has a large scale height and reflects from remote disk regions.

Abstract

We study X-ray spectra from the outburst rise of the accreting black-hole binary MAXI J1820+070. We find that models having the disk inclinations within those of either the binary or the jet imply significant changes of the accretion disk inner radius during the luminous part of the hard spectral state, with that radius changing from $>$100 to $\sim$10 gravitational radii. The main trend is a decrease with the decreasing spectral hardness. Our analysis requires the accretion flow to be structured, with at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and produces strong, narrow, X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with the harder component having a large scale height, located downstream the disk truncation radius, and reflecting mostly from remote parts of the disk. The softer component forms a corona above the disk up to some transition radius. Our findings can explain the changes of the characteristic variability time scales, found in other works, as being driven by the changes of the disk characteristic radii.