Evidence For Advective Flow From Multi-Wavelength Observations Of Nova Muscae

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of black body emissions from the outer region of an accretion disk. We show for self-consistency that scattering effects in this region are not important. The black hole mass (M ≈ 6M⊙), the inclination angle (μ ≈ 0.5) and the distance to the source (D ≈ 5 kpc) have been constrained by optical observations during quiescence (Orosz et al. 1996). Using these values we find that the accretion rate during the peak was Ṁ ≈ 8 × 10 g sec and subsequently decayed exponentially . We define a radiative fraction (f) to be the ratio of the X-ray energy luminosity to the total gravitational power dissipated for a keplerian accretion disk. We find that f ≈ 0.1 and remains nearly constant during the Ultra-soft and Soft spectral states. Thus for these states, the inner region of the accretion disk is advection dominated. f probably increased to ≈ 0.5 during the Hard state and finally decreased to ≈ 0.03 as the source returned to quiescence. We show that advective flow in the disk is optically thick due to high accretion rates during the outburst. This is in contrast to some theoretical models of advection dominated disks which require optical thinness. We speculate that this optically thick advective disk could be the origin of the soft component if copious external cold photons are available. The soft component could also be due to a keplerian non-advective disk which terminates at around R ≈ 30 Schwarzschild radius. However in this case the inner advective flow has to be photon starved. Theoretical models of inner hot accretion disk are generally parameterized in terms of the normalized accretion rate ṁ = Ṁ/ṀEdd, where ṀEdd is the Eddington accretion rate. Our results show that Nova Muscae was in the Ultra-soft state when ṁ ≥ 50, in the Soft state for 50 > ṁ > 2 and