Evidence for Advective Flow from Multiwavelength Observations of Nova MUSCAE

Abstract

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 show for self-consistency that scattering effects in this region are not important. The black hole mass (M~6 M_solar), the inclination angle (μ~0.5), and the distance to the source (D~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 Ṁ~8×10¹⁹ 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 power dissipated for a Keplerian accretion disk. We find that f~0.1 and remains nearly constant during the ultrasoft 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 because of high accretion rates during the outburst. This is in contrast to some theoretical models of advection-dominated disks that 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 nonadvective disk that 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 disks are generally parameterized in terms of the normalized accretion rate ṁ=Ṁ/Ṁ_Edd, where Ṁ_Edd is the Eddington accretion rate. Our results show that Nova Muscae was in the ultrasoft state when ṁ>=50, in the soft state for 50>ṁ>2, and in the hard state for ṁ<=2. Our results constrain present and future theoretical models for the inner regions of accretion disks around black holes. We highlight the need for multiwavelength observations of future black hole novae to confirm the results presented here.