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

We examine the light curve of the 2001 July-August superoutburst of WZ Sagittae. During the decline from maximum light the locally defined decay time increases from ~4 to ~12 days mag -1 over the first ~15 days of the ~25 day superoutburst, as the system faded from m V ≃ 8.5 to m V ≃ 10. The superoutburst is caused by the sudden accretion of ~10 24 g of gas onto the white dwarf, and the deviation from exponentiality in the decay light curve is expected qualitatively during a "viscous decay" in which the dominant mode of depletion of the gas stored in the accretion disk is accretion onto the central object. In other words, as the mass of the accretion disk decreases, the viscous timescale increases. We show that the data are also quantitatively consistent with the theoretical viscous decay time, both calculated via a simple scaling and also from time-dependent calculations, when one adopts standard model parameters for WZ Sge.