http://arxiv.org/abs/2101.04482
We study X-ray spectra acquired during the rise of the outburst of the accreting black-hole binary MAXI J1820+070. We find significant changes of the inner radius of the accretion disk within the luminous part of the hard spectral state, with the disk inner truncation radius changing from more than 100 gravitational radii to $\sim$10. The main trend is a decrease with the decreasing spectral hardness. We find the spectra at photon energies $>$3 keV require the accretion flow to be structured, with at least two components with different spectral slopes. The harder component both dominates the bolometric luminosity and forms a strong narrow core of the X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with a hot Comptonizing plasma present downstream the disk truncation radius, which emits the harder spectral component. However, its reflection is from remote parts of the disk. The second Comptonizing component appears to form a corona above the truncated disk up to some transition radius. Our findings can readily explain the changes of the characteristic variability time scales, found in other works, being driven by the changes of the disk characteristic radii.
A. Zdziarski, M. Dzielak, B. Marco, et. al.
Wed, 13 Jan 21
11/70
Comments: ApJ, submitted
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