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

Abstract

Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state and very high state. We present a self-consistent model of accretion flows around black holes which unifies all of these states except the very high state. The model is an extension of the following paradigm which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) which extends from the black hole horizon to a transition radius rtr, and an outer thin accretion disk that is present beyond rtr. Above the disk is a hot corona which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum. At low mass accretion rates, ˙m < ∼ 0.01 (in Eddington units), the inner ADAF