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

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 ≈12 d at the end of the soft state to ≈7 d at the beginning of the hard state. This factor ∼2 change in the decay time-scale 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.