Non-Steady-State Accretion Disks in X-Ray Novae: Outburst Models for Nova Monocerotis 1975 and Nova Muscae 1991

We have fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova Muscae GS 1991=1124-683) using a non-steady-state accretion-disk model. The model is based on a new solution for a diffusion-type equation for non-steady-state accretion and describes the evolution of a viscous α disk in a binary system after the peak of the outburst, when the matter in the disk is totally ionized. The accretion rate in the disk decreases according to a power law. We derive formulas for the accretion rate and effective temperature of the disk. The model has three free input parameters: the mass of the central object M, the turbulence parameter α, and the normalization parameter δt. The results of the modeling are compared with the observed X-ray and optical B and V light curves. The estimates for the turbulence parameter α are similar: 0.2 0.4 for A 0620-00 and 0.45 0.65 for GS 1124-683, suggesting a similar nature for the viscosity in the accretion disks around the compact objects in these sources. We have also derived the distances to these systems as functions of the masses of their compact objects.