X-ray irradiation in XTE J1817-330 and the inner radius of the truncated disc in the hard state

TL;DR

This study reanalyzes XTE J1817-330 data, providing strong evidence that the inner accretion disc recedes during the transition to the low/hard state, supporting the truncated disc model despite previous challenges.

Contribution

It demonstrates that the low/hard state disc recedes as the source transitions, and accounts for irradiation effects that influence radius estimates, reinforcing the truncated disc model.

Findings

Disc begins to recede during the soft to hard transition.

Irradiation causes underestimation of the disc radius by a factor of 2-3.

Inner disc radius in the low/hard state is at least 6-8 times larger than in the high/soft state.

Abstract

The key aspect of the very successful truncated disc model for the low/hard X-ray spectral state in black hole binaries is that the geometrically thin disc recedes back from the last stable orbit at the transition to this state. This has recently been challenged by direct observations of the low/hard state disc from CCD data. We reanalyze the Swift and RXTE campaign covering the 2006 outburst of XTE J1817-330 and show that these data actually strongly support the truncated disc model as the transition spectra unambiguously show that the disc begins to recede as the source leaves the disc dominated soft state. The disc radius inferred for the proper low/hard state is less clear-cut, but we show that the effect of irradiation from the energetically dominant hot plasma leads to an underestimate of the disc radius by a factor of 2-3 in this state. This may also produce the soft excess reported in some hard-state spectra. The inferred radius becomes still larger when the potential difference in stress at the inner boundary, increased colour temperature correction from incomplete thermalization of the irradiation, and loss of observable disc photons from Comptonization in the hot plasma are taken into account. We conclude that the inner disc radius in XTE J1817-330 in the low/hard spectral state is at least 6-8 times that seen in the disc dominated high/soft state, and that recession of the inner disc is the trigger for the soft--hard state transition, as predicted by the truncated disc models.