An optically thick inner corona geometry for the Very High State Galactic Black Hole XTE J1550-564

TL;DR

This paper investigates the inner accretion disc geometry in the Very High State of a galactic black hole, proposing a model where the disc's emissivity is altered by a corona, explaining observed spectral features.

Contribution

It introduces a coupled disc-corona model that accounts for low disc temperatures and high luminosities without requiring large disc truncation, advancing understanding of accretion physics.

Findings

The corona is likely optically thick and covers a large inner disc region.

Standard disc models require a large increase in inner radius to fit the data.

A modified disc emissivity model explains observations with minimal disc truncation.

Abstract

(truncated version) The X-ray spectra of Galactic binary systems in the very high state show both strong disk emission and a strong, steep tail to high energies. We use simultaneous optical-ASCA-RXTE data from the black hole transient XTE J1550-564 as a specific example, and show that these have disc spectra which are significantly lower in temperature than those seen from the same source at the same luminosity when in the disc dominated state. If these give a true picture of the disc then either the disc emissivity has reduced, and/or the disc truncates above the last stable orbit. However, it is often assumed that the tail is produced by Compton scattering, in which case its shape in these spectra requires that the Comptonising region is marginally optically thick (tau~2-3), and covers a large fraction of the inner disc. This will distort our view of the disc. We build a theoretical model of a Comptonising corona over an inner disc, and fit this to the data, but find that it still requires a large increase in inner disc radius for a standard disc emissivity. Instead it seems more probable that the disc emissivity changes in the presence of the corona. We implement the specific inner disc-corona coupling model of Svensson & Zdziarski (1994) and show that this can explain the low temperature/high luminosity disc emission seen in the very high state with only a small increase in radius of the disc. While this inferred disc truncation is probably not significant given the model uncertainties, it is consistent with the low frequency QPO and gives continuity of properties with the low/hard state spectra.