  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.
