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 V404Cyg in quiescence. We find that in the energy range 0.7 to 8.5 keV the spectrumis 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 flowhas two components: (1) an outer thin disk with a small annular extent, and (2)a large interior region where the flow is advection-dominated. Nearly all theradiation in the infrared, optical, UV and X-ray bands is from theadvection-dominated zone; the thin disk radiates primarily in the infraredwhere it contributes about ten percent of the observed flux. The spectrum wecalculate with this model is in excellent agreement with the ASCA X-ray datapresented here, as well as with previous optical data. Moreover, the fit isvery insensitive to the choice of parameters such as black hole mass, orbitalinclination, viscosity coefficient $\alpha$, and magnetic field strength. Weconsider the success of the model to be strong support for theadvection-dominated accretion paradigm, and further evidence of the black holenature of V404 Cyg. We discuss strategies whereby systems withadvection-dominated accretion could be used to prove the reality of eventhorizons in black holes.Comment: Final version to appear in The Astrophysical Journal. Includes two new subsections, 2 new Figures and 2 new Table