Black Hole X-Ray Transients

Irradiation models for soft X-ray transients (SXTs) are considered. These require the presence of hard X-rays for the outburst to be triggered. If the accreting object is a neutron star this flux is likely to arise in a boundary layer. For black-hole transients recent work by White and Lightman (1989) shows that two-temperature behavior is likely in the inner disk at accretion rates higher than a critical value M(dot)TT, thus providing the required hard X-rays. M(dot)TT is a function of the disk viscosity parameter alpha and the mass of the accreting object. The quiescent accretion rate of the black-hole candidate SXT A0620 - 00 is probably slightly lower than M(dot)TT and its disk is in a one-temperature state. The slow expansion of the secondary increases the mass transfer rate and when a two-temperature structure arises the system goes into outburst. This model requires alpha at about 0.1. Compared with neutron-star SXTs having similar binary parameters, black hole SXTs are likely to be brighter both in quiescence and in outburst. Their outbursts are likely to be shorter, but the quiescent intervals are expected to be similar to those in the neutron-star case.