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

Abstract

We examine the AAVSO light curve of U Gem from 1908 to 2002, with particular focus on the 1985 October 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 timescale during the initial slow decay. This rate is ~26 ± 6 days mag-1. Using estimates of the rate of accretion during outburst taken from Froning et al., one can show that ~1024 g of gas was accreted onto the white dwarf during the outburst, which constrains the surface density in the outer accretion disk to be ~600 g cm-2. The observed timescale for decay is consistent with that expected in U Gem, given its orbital period and disk mass at the time the outburst began. The data are not of sufficient quality to be able to ascertain a deviation from exponentiality in the decay light curve (as in the SU UMa stars' superoutbursts). When coupled with the time inferred from the (short orbital period) SU UMa stars, the U Gem timescale lends support to the standard model in which the decays in dwarf novae can either be or thermal, with the ratio between them being roughly h/r, where h is the vertical pressure scale height in the disk. Indeed, dwarf novae are the only systems for which one can be reasonably certain of the identification of viscous and thermal decays.