Hubble Space Telescope observations of ultraviolet oscillations in WZ Sagittae during the decline from outburst

We present a time series analysis of Hubble Space Telescope observations of WZ Sge obtained in 2001 September, October, November, and December as WZ Sge declined from its 2001 July superoutburst. Previous analysis of these data showed the temperature of the white dwarf decreased from similar to29,000 to similar to18,000 K. In this study we binned the spectra over wavelength to yield ultraviolet light curves at each epoch that were then analyzed for the presence of the well-known 27.87 and 28.96 s oscillations. We detect the 29 s periodicity at all four epochs, but the 28 s periodicity is absent. The origin of these oscillations has been debated since their discovery in the 1970s, and competing hypotheses are based on either white dwarf non-radial g-mode pulsations or magnetically channelled accretion onto a rotating white dwarf. By analogy with the ZZ Ceti stars, we argue that the nonradial g-mode pulsation model demands a strong dependence of pulse period on the white dwarf's temperature. However, these observations show that the 29 s oscillation is independent of the white dwarf's temperature. Thus, we reject the white dwarf nonradial g-mode pulsation hypothesis as the sole origin of the oscillations. It remains unclear if magnetically funneled accretion onto a rapidly rotating white dwarf (or belt on the white dwarf) is responsible for producing the oscillations. We also report the detection of a quasi-periodic oscillations (QPO) with period similar to18 s in the September light curve. The amplitudes of the 29 s oscillation and the QPO vary erratically on short timescales and are not correlated with the mean system brightness nor with each other.