Evidence for changes in the radiative efficiency of transient black hole X-ray binaries

TL;DR

This study analyzes long-term X-ray light curves of six transient black hole binaries, revealing a change in decay timescales around state transitions that supports the presence of radiatively inefficient accretion in the hard state.

Contribution

It provides observational evidence for a change in radiative efficiency during state transitions in black hole X-ray binaries, supporting models of radiatively inefficient accretion flows.

Findings

Decay timescales decrease from ~12 days to ~7 days at state transition

Change in decay timescale supports radiatively inefficient accretion in the hard state

Exponential decay patterns observed in X-ray light curves

Abstract

We have used pointed RXTE data to examine the long-term X-ray light curves of six transient black hole X-ray binaries during their decay from outburst to quiescence. In most cases there is a period of exponential decay as the source approaches the soft-to-hard state transition, and another period of exponential decay following this transition as the source decays in the hard state. The e-folding times change around the time of the state transition, from typically approx 12 days at the end of the soft state to approx 7 days at the beginning of the hard state. This factor ~2 change in the decay timescale is expected if there is a change from radiatively efficient emission in the soft state to radiatively inefficient emission in the hard state, overlying an exponential decay in the mass accretion rate. This adds support to the idea that the X-ray emitting region is governed by radiatively inefficient accretion (such as an advection-dominated or jet-dominated accretion flow) during the fading hard state.