Disc Instabilities in Soft X-ray Transients

Abstract

I briefly review the theory of soft X-ray transient systems. Irradiation of the accretion disc faces by the central X-ray source determines both the occurrence and the nature of the outbursts, in particular forcing these to be long viscous events and producing exponential decays at short orbital periods. Soft X-ray transients constitute the majority of LMXBs, persistent systems being largely confined to a subset of neutron-star LMXBs with periods P < 2 d. It appears that LMXBs very frequently contain nuclear-evolved companions, even at short orbital periods. In long-period transients (P > 20 d) the outburst recurrence times must become extremely long (> 1500 yr). The outbursts are highly super-Eddington, markedly reducing the accretion efficiency. Spinup of a neutron-star primary to millisecond periods probably cannot occur for orbital periods > 200 d, in agreement with observations of binary pulsars.