Evolution of the Accretion Flow in Nova MUSCAE 1991

We identify the X-ray-reflected component in the Ginga spectra of Nova Muscae 1991, a black hole transient system used as the prototype for the recent model of Esin, McClintock, & Narayan based on advection-dominated disk solutions. We see that the reflected spectrum is generally significantly relativistically smeared and use this, together with the amplitude of reflection, to track the innermost extent of the accretion disk. The optically thick disk switches from being highly ionized to nearly neutral during the transition from high to low state, and the inner radius of the disk moves outward during the low-state decline.

Qualitatively, this overall trend is compatible with the Esin et al. model, but quantitatively, the retreat of the inner disk during the high- to low-state transition is much slower than predicted. The hard (low-state) spectra are not produced solely by an optically thin accretion flow: optically thick material within ~20-100R_g is generally present.