Revealing accretion onto black holes: X-ray reflection throughout three outbursts of GX 339-4

TL;DR

This study systematically analyzes X-ray reflection spectra during three outbursts of GX 339-4, revealing how accretion geometry and spectral properties evolve across different black hole states.

Contribution

It provides the largest self-consistent analysis of reflection spectra over multiple outbursts, elucidating accretion geometry changes during state transitions.

Findings

Reflection is weak in the hard state and increases with luminosity.

Reflection dominates the spectrum in the soft state and increases as the source decays.

Evolution of spectral parameters supports inner disc truncation and coronal height changes.

Abstract

Understanding the dynamics behind black hole state transitions and the changes they reflect in outbursts has become long-standing problem. The X-ray reflection spectrum describes the interaction between the hard X-ray source (the power-law continuum) and the cool accretion disc it illuminates, and thus permits an indirect view of how the two evolve. We present a systematic analysis of the reflection spectrum throughout three outbursts (500+ observations) of the black hole binary GX 339-4, representing the largest study applying a self-consistent treatment of reflection to date. Particular attention is payed to the coincident evolution of the power-law and reflection, which can be used to determine the accretion geometry. The hard state is found to be distinctly reflection weak, however the ratio of reflection to power-law gradually increases as the source luminosity rises. In contrast the reflection is found dominate the power-law throughout most of the soft state, with increasing supremacy as the source decays. We discuss potential dynamics driving this, favouring inner disc truncation and decreasing coronal height for the hard and soft states respectively. Evolution of the ionisation parameter, power-law slope and high-energy cut-off also agree with this interpretation.