Self-similar viscous decay in dwarf-nova outbursts

Viscous decay in dwarf-nova outbursts is studied using self-similar solutions. We find that the brightness temperature distribution of the dwarf nova Z Cha in outburst, obtained by the eclipse mapping methods, is well-reproduced by a self-similar solution. The temperature distribution of this solution is close to that of the steady disc at inner radii, but the mass-flow rate through the disc gradually decreases with increase in radius and becomes negative (mass outflow) beyond a certain radius. The original self-similar solution with constant angular momentum, however, produces longer decay times than the observed values. To avoid this difficulty, we need to assign large values for the viscosity parameter, or to efficiently remove angular momentum from hot portions of the disc by some mechanism, such as the thermal instability predicted by the disc instability model. (Author)