A Population of Faint Nontransient Low‐Mass Black Hole Binaries

We study the thermal and viscous stability of accretion flows in Low MassBlack Hole Binaries (LMBHBs). We consider a model in which an inneradvection-dominated accretion flow (ADAF) is surrounded by a geometrically thinaccretion disk, the transition between the two zones occurring at a radiusR_tr. In all the known LMBHBs, R_tr appears to be such that the outer diskscould suffer from a global thermal-viscous instability. This instability islikely to cause the transient behavior of these systems. However, in mostcases, if R_tr were slightly larger than the estimated values, the systemswould be globally stable. This suggests that a population of faint persistentLMBHBs with globally stable outer disks could be present in the Galaxy. SuchLMBHBs would be hard to detect because they would lack large amplitudeoutbursts, and because their ADAF zones would have very low radiativeefficiencies, making the systems very dim. We present model spectra of suchsystems covering the optical and X-ray bands.Comment: LateX, 37 pages, 11 figures; Accepted for publication in The Astrophysical Journa