Why Low-Mass Black Hole Binaries Are Transient

We consider transient behavior in low-mass X-ray binaries (LMXBs). In short-period neutron star systems (orbital period <~1 day) irradiation of the accretion disk by the central source suppresses this behavior except at very low mass transfer rates. Formation constraints, however, imply that a significant fraction of these neutron star systems have nuclear-evolved main-sequence secondaries and thus mass transfer rates low enough to be transient. But most short-period low-mass black hole systems will form with unevolved main-sequence companions and have much higher mass transfer rates. The fact that essentially all of them are nevertheless transient shows that irradiation is weaker, which is a direct consequence of the fundamental black hole property--the lack of a hard stellar surface.