Why Low‐Mass Black Hole Binaries Are Transient

We consider transient behavior in low-mass X-ray binaries. In short-periodneutron-star systems (orbital period less than ~ 1d) irradiation of theaccretion disk by the central source suppresses this except at very low masstransfer rates. Formation constraints however imply that a significant fractionof these neutron star systems have nuclear-evolved main-sequence secondariesand thus mass transfer rates low enough to be transient. But most short-periodlow-mass black-hole systems will form with unevolved main-sequence companionsand have much higher mass transfer rates. The fact that essentially all of themare nevertheless transient shows that irradiation is weaker, as a directconsequence of the fundamental black-hole property - the lack of a hard stellarsurface.Comment: 13 pages (including 3 figures); accepted for publication in Ap