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

TL;DR

This paper demonstrates that soft X-ray excesses observed in the low-hard state of black hole binaries can naturally arise from a truncated disk interacting with a hot inner flow, challenging the idea of a highly truncated disk.

Contribution

It introduces a model where ion-bombardment heats the disk surface, producing soft X-ray excesses consistent with observations in the low-hard state.

Findings

Soft X-ray excesses can be produced by a hot surface layer on the disk.

The model explains observations without requiring the disk to extend to the innermost stable orbit.

Interactions between the hot flow and the truncated disk are key to the spectral features.

Abstract

Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127) in the low-hard state (L_X/L_Edd ~ 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 standard accretion disk, producing a hot (kT_e ~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.