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

A J Eckersall,S Vaughan,G A Wynn

doi:10.1093/mnras/stv739

A J Eckersall, S Vaughan + Show 1 more

Open Access

https://doi.org/10.1093/mnras/stv739

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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.

Highlights

Low-mass X-ray binaries (LMXBs) comprise a black hole (BH) or neutron star (NS), often called the primary, that accretes gas from a secondary star with a mass M2 ∼< 1.5 M⊙
If radiative efficiency is a power-law function of the central mass accretion rate, η ∝ Mβ, and the mass accretion rate follows an exponential decay with e-folding time τm, the X-ray luminosity should follow LX = A exp(−t(1 + β)/τm)
Visual inspection of the light curves, for each source we identified two regions of decay separated by the soft– hard state transition. (During this transition – spanning intermediate states – the light curve often stays approximately constant or even rises slightly)

Summary

INTRODUCTION

Low-mass X-ray binaries (LMXBs) comprise a black hole (BH) or neutron star (NS), often called the primary, that accretes gas from a secondary star ( called the companion, or donor) with a mass M2 ∼< 1.5 M⊙. The soft state (sometimes known as the thermal-dominated state) is generally seen near the peak of the outburst and is characterized by an X-ray spectrum dominated by thermal emission from the accretion disc with a temperature Tpeak ∼ 1 keV (Remillard & McClintock 2006), and a weak, nonthermal tail of emission to higher energies. When the mass accretion rate is decaying exponentially, and the emission is dominated by a radiatively efficient accretion disc, the X-ray luminosity should decay with the same e-folding time: LX ∝ M ∝ exp(−t/τm). As discussed in Homan et al (2005), if there is a change from a radiatively efficient to a radiatively inefficient accretion regime during the period of exponential decay in the mass accretion rate, there should be a decrease in the e-folding time of the X-ray luminosity during the decay. We test this hypothesis using some of the best RXTE monitoring data of XRB outbursts

Sample definition

RXTE observations

Swift Observations

The Model

Results

DISCUSSION