ADVECTION-DOMINATED ACCRETION AND THE SPECTRAL STATES OF BLACK-HOLE X-RAY BINARIES - APPLICATION TO NOVA-MUSCAE-1991

Black hole X-ray binaries (BHXBs) are known to display five distinct s pectral states. In order of increasing luminosity these are the quiesc ent state, low state, intermediate state, high state, and very high st ate. We present a self-consistent model of accretion flows around blac k holes that unifies all of these states except the very high state. T he model is an extension of the following paradigm, which has been app lied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) that extends from the black hole horizon to a transition radius r(tr), and an outer thin accretion disk that is present beyond r(tr). Above the d isk is a hot corona, which is a continuation of the inner ADAF. The mo del consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and cor ona, and the radiation processes that produce the observed spectrum. A t low mass accretion rates, (m) over dot less than or similar to 0.01 (in Eddington units), the inner ADAF zone in the model radiates extrem ely inefficiently, and the outer thin disk is restricted to large radi i (r(tr) similar to 10(2)-10(4), in Schwarzschild units). The luminosi ty therefore is low, and this configuration is identified with the qui escent state. For (m) over dot greater than or similar to 0.01 and up to a critical value (m) over dot(crit) similar to 0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value of (m) over dot(crit) depends on the v iscosity parameter alpha ((m) over dot(crit) similar to 1.3 alpha(2); the paper assumes alpha = 0.25). For values of (m) over dot > (m) over dot(crit) and up to a second critical value about 10% higher, the ADA F progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, when (m) over dot is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified sc enario agree well with observations of the quiescent, low, intermediat e, and high states. Moreover, the model provides a natural explanation for the low state to high state transition in BHXBs. We also make a t entative proposal for the very high state, but this aspect of the mode l is less secure. An important feature of the model is that it is esse ntially parameter free. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model r eproduces the observed light curves and spectra surprisingly well and makes a number of predictions that can be tested by observations of ot her BHXBs.