Soft X-ray components in the hard state of accreting black holes

Abstract

Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127) in the low-hard state ($L_{\rm{X}}/L_{\rm{Edd}}$ $\simeq$ 0.003-0.05) suggest the presence of a cool accretion disk very close to the innermost stable orbit of the black hole. This runs counter to models of the low-hard state in which the cool disk is truncated at a much larger radius. We study the interaction between a moderately truncated disk and a hot inner flow. Ion-bombardment heats the surface of the disk in the overlap region between a two-temperature advection-dominated accretion flow and a standard accretion disk, producing a hot ($kT_{\rm{e}}$ $\simeq$ 70 keV) layer on the surface of the cool disk. The hard X-ray flux from this layer heats the inner parts of the underlying cool disk, producing a soft X-ray excess. Together with interstellar absorption these effects mimic the thermal spectrum from a disk extending to the last stable orbit. The results show that soft excesses in the low-hard state are a natural feature of truncated disk models.