Evidence of Light-Bending Effects and its implication for spectral state transitions

TL;DR

This study analyzes X-ray observations of a black hole binary to explore how light bending effects influence spectral state transitions, revealing that coronal height and gravitational light bending impact accretion flow behavior.

Contribution

It provides evidence that coronal height and light bending effects are key factors driving spectral state transitions in black hole binaries, supported by spectral and timing analysis.

Findings

Inner disk radius remains constant during state transitions.

Reflection fraction increases sharply at certain state transitions.

High-frequency QPOs correlate with reflection fraction.

Abstract

It has long been speculated that the nature of the hard X-ray corona may be an important second driver of black hole state transitions, in addition to the mass accretion rate through the disk. However, a clear physical picture of coronal changes has not yet emerged. We present results from a systematic analysis of Rossi X-ray Timing Explorer observations of the stellar mass black hole binary XTE J1650-500. All spectra with significant hard X-ray detections were fit using a self-consistent, relativistically-blurred disk reflection model suited to high ionization regimes. Importantly, we find evidence that both the spectral and timing properties of black hole states may be partially driven by the height of the X-ray corona above the disk, and related changes in how gravitational light bending affects the corona--disk interaction. Specifically, the evolution of the power-law, thermal disk, and relativistically--convolved reflection components in our spectral analysis indicate that: (1) the disk inner radius remains constant at \rin$=1.65\pm0.08 GM/c^2$ (consistent with values found for the ISCO of XTE J1650-500 in other works) throughout the transition from the \textit{brighter} phases of the low-hard state to the intermediate states (both the hard-intermediate and soft-intermediate), through to the soft state and back; (2) the ratio between the observed reflected X-ray flux and power-law continuum (the "reflection fraction", R) increases sharply at the transition between the hard-intermediate and soft-intermediate states ("ballistic" jets are sometimes launched at this transition); (3) both the frequency and coherence of the high-frequency quasi-periodic oscillations (QPOs) observed in XTE J1650-500 increase with R. We discuss our results in terms of black hole states and the nature of black hole accretion flows across the mass scale.