A Population of Faint Nontransient Low‐Mass Black Hole Binaries

Abstract

We study the thermal and viscous stability of accretion flows in low-mass black hole binaries (LMBHBs). We consider a model in which an inner advection-dominated accretion flow (ADAF) is surrounded by a geometrically thin accretion disk, the transition between the two zones occurring at a radius R-tr. In all known LMBHBs, R-tr appears to be such that the outer disks could suffer from global thermal-viscous instability. This instability is likely to cause the transient behavior of these systems. However, in most cases, if R-tr were slightly larger than the estimated values, the systems would be globally stable. This suggests that a population of faint persistent LMBHBs with globally stable outer disks could be present in the Galaxy. Such LMBHBs would be hard to detect because they would lack large-amplitude outbursts and because their ADAF zones would have very low radiative efficiencies, making the systems very dim. We present model spectra of such systems covering the optical and X-ray bands.