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 similar to4 to similar to2 days mag(-) over the first similar to 15 days of the similar to 25 day superoutburst, as the system faded from to. The superoutburst is caused by the m(V) similar or equal to 8.5 m(V) similar or equal to 10 sudden accretion of similar to 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.