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 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, ∼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 rate of
\sim10^{-11}\msyr in A0620-00 and Nova Mus, and \sim{\rm
few}\times10^{-10}\msyr in V404 Cyg. The best fit to the data is obtained for
a viscosity parameter α∼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
γ-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 viscousComment: 32 Pages, 6 Figures included, Compressed Postscript, To Appear in
Astrophysical Journa