Astronomy and Astrophysics – Astrophysics
Scientific paper
1996-10-02
Astronomy and Astrophysics
Astrophysics
Final version to appear in The Astrophysical Journal. Includes two new subsections, 2 new Figures and 2 new Tables
Scientific paper
10.1086/304134
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 to 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 with a small annular extent, and (2) a large interior region where the flow 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 about ten percent 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 insensitive to the choice of parameters such as black hole mass, orbital inclination, viscosity coefficient $\alpha$, and magnetic field strength. We consider the success of the model to be strong support for the advection-dominated accretion paradigm, and further evidence of the black hole nature of V404 Cyg. We discuss strategies whereby systems with advection-dominated accretion could be used to prove the reality of event horizons in black holes.
Barret Didier
McClintock Jeffrey E.
Narayan Ramesh
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