On the complex disc-corona interactions in the soft spectral states of Soft X-ray Transients

Accreting black holes show a complex and diverse behaviour in their soft spectral states. Although these spectra are dominated by a soft, thermal component which almost certainly arises from an accretion disc, there is also a hard X-ray tail indicating that some fraction of the accretion power is instead dissipated in hot, optically thin coronal material. During such states, best observed in the early outburst of Soft X-ray Transients, the ratio of power dissipated in the hot corona to that in the disc can vary from ~0 (pure disc accretion) to ~1 (equal power in each). Here we present results of spectral analyses of a number of sources, demonstrating the presence of complex features in their energy spectra. Our main findings are: (1) the soft components are not properly described by a thermal emission from accretion discs: they are appreciably broader than can be described by disc blackbody models even including relativistic effects, and (2) the spectral features near 5-9 keV commonly seen in such spectra can be well described by reprocessing of hard X-rays by optically thick, highly ionized, relativistically moving plasma.