A new model for black hole soft X-ray transients in quiescence

We present models of the soft X-ray transients, A0620-00, V404 Cyg, and X-ray Nova Mus 1991, in quiescence. In each source, we postulate that there is an outer region, extending outward from about 3000 Schwarzschild radii, where the accretion flow is in the form of a standard thin disk. The outer disk produces most of the radiation we observe in the infrared, optical, and UV bands. We propose that the disk undergoes an instability at its inner edge, perhaps by the mechanism described recently by Meyer & Meyer-Hofmeister (1994) for cataclysmic variables. The accreting gas is thereby converted into a hot corona which flows into the black hole as a nearly virial two-temperature flow. We describe the hot inner flow by means of a recently discovered stable solution of optically thin advection-dominated accretion. In this flow, most of the thermal energy released by viscous dissipation is advected into the black hole and only a small fraction, similar to 10-(4)-10(-3), of the energy is radiated. The radiation is in the form of Comptonized synchrotron and bremsstrahlung emission and has a broad spectrum extending from optical to soft gamma-rays. The models we present are consistent with all the available data in the three sources. In particular, the X-ray emission from the hot inner flow fits the observed flux and spectral index of A0620-00. We derive a mass accretion fate of similar to 10(-11) M(.) yr(-1) in A0620-00 and Nova Mus, and similar to few times 10(-10) M(.) yr(-1) in V404 Cyg. The best fit to the data is obtained for a viscosity parameter alpha similar to 0.1-0.3 in the hot flow. The models predict that all three sources must have substantial flux in hard X-rays and soft gamma-rays. This prediction is testable in the case of V404 Cyg with current instruments. A necessary feature of our proposal is that most of the viscous energy released in the accretion is advected into the black hole without being radiated. The success of the models in fitting the observations therefore indicates that the accreting stars have event horizons and therefore confirms the black hole nature of these stars.