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 .

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 \approx 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 { \dot { m } } = { \dot { M } } / { \dot { M } } _ { Edd } , where { \dot { M } } _ { Edd } is the Eddington accretion rate .
Our results show that Nova Muscae was in the Ultra-soft state when { \dot { m } } \geq 50 , in the Soft state for 50 > { \dot { m } } > 2 and in the Hard state for { \dot { m } } \leq 2 .

Our results constrain present and future theoretical models for the inner regions of accretion disks around black holes .
We highlight the need for multi-wavelength observations of future black hole novae to confirm the results presented here .