ON THE DECAY OF OUTBURSTS IN DWARF NOVAE AND X-RAY NOVAE

We perform computations using a time-dependent model for the accretion disk limit-cycle mechanism to examine the decay of the optical light following the peak of a dwarf nova outburst. We present the results of a parameter study of the physical input variables which affect the de cay rate. In the model, the decay is brought about by a cooling transi tion front which begins at large radii in the disk and moves inward. T he nature of the decay is strongly influenced by the radial dependence of the accretion disk viscosity parameter alpha. To obtain exponentia l decays for typical dwarf nova parameters, we require alpha proportio nal to r(epsilon 0), where epsilon(0) similar or equal to 0.3-0.4. Thi s is consistent with the dependence inferred by Mineshige and Wood usi ng observations of the radial variation of brightness temperature of d warf novae in quiescence. The exact value of epsilon(0) which produces exponential decays depends on factors such as the mass of the accreti ng star and the inner radius of the accretion disk. Therefore, the obs erved ubiquity of exponential decays in two different types of systems (dwarf novae and X-ray novae) leads us to believe that alpha is an un natural scaling for the viscosity. The physics of the cooling transiti on front must be self-regulating in that the timescale [-partial deriv ative In Sigma(r)/partial derivative t](-1) (where Sigma is the surfac e density) for mass extraction across the front remains constant. This may be consistent with a scaling alpha proportional to (h/r)(n), wher e h is the local disk semi-thickness and n similar to 1-2. As regards the speed of the cooling front, we find v(F)(r) proportional to r(p), where p similar to 3 at large radii, with an abrupt transition to p si milar to 0 at some smaller radius. The r(3) dependence is much steeper than has been found by previous workers and appears to result from th e strong variation of specific heat within the cooling front when the front resides at a large radius in the disk. The outflow of disk mater ial across the cooling front causes a significant departure of dln T-e ff/d ln r from the standard value of -0.75 (expected from steady state accretion) within about 0.2 dex in radius of the break associated wit h the cooling front-T-eff similar to 10(3.9) K (r/10(10) cm)(-0.1). Th ese effects should be observable with eclipse mapping. Finally, it app ears that the relatively slow decay rate for the optical flux in the 1 975 outburst of A0620-00 can be accounted for if the primary is a simi lar to 10 M. black hole.