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 Rtr. In all known LMBHBs, Rtr 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 Rtr 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.