Evidence for advective flow from multiwavelength observations of Nova Muscae

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of blackbody emissions from the outer region of an accretion disk. We s how for self-consistency that scattering effects in this region are not imp ortant. The black hole mass (M approximate to 6 M-circle dot), the inclinat ion angle (mu approximate to 0.5), and the distance to the source (D approx imate to 5 kpc) have been constrained by optical observations by Orosz and coworkers during quiescence. Using these values we find that the accretion rate during the peak was (M) over dot approximate to 8 x 10(19) g s(-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 powe r dissipated for a Keplerian accretion disk. We find that f approximate to 0.1 and remains nearly constant during the ultrasoft and soft spectral stat es. Thus, for these states, the inner region of the accretion disk is advec tion-dominated; f probably increased to approximate to 0.5 during the hard state and finally decreased to approximate to 0.03 as the source returned t o quiescence. We show that advective flow in the disk is optically thick be cause of high accretion rates during the outburst. This is in contrast to s ome theoretical models of advection-dominated disks that require optical th inness. 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 nonadvective disk tha t terminates at around R approximate to 30 Schwarzschild radius. However, i n this case the inner advective flow has to be photon-starved. Theoretical models of inner hot accretion disks are generally parameterized in terms of the normalized accretion rate (m) over dot = (M) over dot/(M) over dot(Edd ), where (M) over dot(Edd) is the Eddington accretion rate. Our results sho w that Nova Muscae was in the ultrasoft state when (m) over dot greater tha n or equal to 50, in the soft state for 50 > (m) over dot > 2, and in the h ard state for (m) over dot less than or equal to 2. Our results constrain p resent and future theoretical models for the inner regions of accretion dis ks around black holes. We highlight the need for multiwavelength observatio ns of future black hole novae to confirm the results presented here.