The 2001 Superoutburst of WZ Sagittae: A Clue to the Dynamics of Accretion Disks

We examine the light curve of the July-August 2001 superoutburst of WZSagittae. During the decline from maximum light the locally defined decay timeincreases from ~4 d/mag to ~12 d/mag over the first ~15 d of the ~25 dsuperoutburst, as the system faded from m_V~8.5 to m_V~10. The superoutburst iscaused by the sudden accretion of ~10^{24} g of gas onto the white dwarf, andthe deviation from exponentiality in the decay light curve is expectedqualitatively during a ``viscous decay'' in which the dominant mode ofdepletion of the gas stored in the accretion disk is accretion onto the centralobject. In other words, as the mass of the accretion disk decreases, theviscous time scale increases. We show that the data are also quantitativelyconsistent with the theoretical viscous decay time, both calculated via asimple scaling and also from time dependent calculations, when one adoptsstandard model parameters for WZ Sge.Comment: 13 pages, 2 figure