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 ($M \approx 6 M_\odot$), the inclination angle ($\mu \approx 0.5$) and the distance to the source ($D \approx 5$ 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 ${\dot M} \approx 8 \times 10^{19}$ g sec$^{-1}$ 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 $f \approx 0.1$ 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 $\approx 0.5$ during the Hard state and finally decreased to $\approx 0.03$ as the source returned to quiescence.