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 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, $\sim10^{-4}-10^{-3}$, 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 $\alpha\sim0.1-0.3$ 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 viscousComment: 32 Pages, 6 Figures included, Compressed Postscript, To Appear in Astrophysical Journa

On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?

Type I X-ray bursts are very common in neutron star X-ray binaries, but no Type I burst has been seen in the dozen or so binaries in which the accreting compact star is too massive to be a neutron star and therefore is identified as a black hole candidate. We have carried out a global linear stability analysis of the accumulating fuel on the surface of a compact star to identify the conditions under which thermonuclear bursts are triggered. Our analysis, which improves on previous calculations, reproduces the gross observational trends of bursts in neutron star systems. It further shows that, if black hole candidates have surfaces, they would very likely exhibit instabilities similar to those that lead to Type I bursts on neutron stars. The lack of bursts in black hole candidates is thus significant, and indicates that these objects have event horizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of Astrophysical Journal Letters, significant changes to the methods, results unchange