Advection‐Dominated Accretion and the Spectral States of Black Hole X‐Ray Binaries: Application to Nova Muscae 1991

We present a self-consistent model of accretion flows which unifies fourdistinct spectral states observed in black hole X-ray binaries: quiescent, low,intermediate and high states. In the quiescent, low and intermediate states,the flow consists of an inner hot advection-dominated part extending from theblack hole horizon to a transition radius and an outer thin disk. In the highstate the thin disk is present at all radii. The model is essentiallyparameter-free and treats consistently the dynamics of the accretion flow, thethermal balance of the ions and electrons, and the radiation processes in theaccreting gas. With increasing mass accretion rate, the model goes through asequence of stages for which the computed spectra resemble very wellobservations of the four spectral states; in particular, the low-to-high statetransition observed in black hole binaries is naturally explained as resultingfrom a decrease in the transition radius. We also make a tentative proposal forthe very high state, but this aspect of the model is less secure. We test the model against observations of the soft X-ray transient NovaMuscae during its 1991 outburst. The model reproduces the observed lightcurvesand spectra surprisingly well, and makes a number of predictions which can betested with future observations.Comment: 68 pages, LaTeX, includes 1 table (forgotten in the previous version) and 14 figures; submitted to The Astrophysical Journa