ADVECTION-DOMINATED ACCRETION MODEL OF THE BLACK-HOLE V404 CYGNI IN QUIESCENCE

We have analyzed archival ASCA data on the soft X-ray transient source V404 Cyg in quiescence. We find that in the energy range 0.7-8.5 keV the spectrum is a hard power law with a photon spectral index between 1.8 and 2.6 (90% confidence limits). We present a model of V404 Cyg in which the accretion flow has two components: (1) an outer thin disk w ith a small annular extent and (2) a large interior region where the f low is advection dominated. Nearly all the radiation in the infrared, optical, UV, and X-ray bands is from the advection-dominated zone; the thin disk radiates primarily in the infrared where it contributes abo ut 10% of the observed flux. The spectrum we calculate with this model is in excellent agreement with the ASCA X-ray data presented here, as well as with previous optical data. Moreover, the fit is very insensi tive to the choice of parameters such as black hole mass,orbital incli nation, viscosity coefficient a, and magnetic field strength. We consi der the success of the model to be strong support for the advection-do minated accretion paradigm, and further evidence of the black hole nat ure of V404 Cyg. We discuss strategies whereby systems with advection- dominated accretion could be used to prove the reality of event horizo ns in black holes.