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 lesssim1 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.