The October 1985 Long Outburst of U Geminorum: Revealing the Viscous Time Scale in Long Orbital Period Dwarf Novae

We examine the AAVSO light curve of U Geminorum from 1908 to 2002, with particular focus on the October 1985 outburst. This outburst was longer than any other seen in U Gem by about a factor of 2, and appears to be unique among all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr in that one can measure the decay time scale during the initial slow decay. This rate is ~26+-6 d/mag. Using estimates of the rate of accretion during outburst taken from Froning et al., one can show that ~1e24 g of gas was accreted onto the white dwarf during the outburst. When coupled with the viscous time inferred from the (short orbital period) SU UMa stars, the U Gem viscous time scale lends support to the standard model in which the decays in dwarf novae can either be viscous or thermal, with the ratio between them being roughly h/r where h is the vertical pressure scale height in the disk.