Discovery of 15 second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst

C. Knigge, University of Southampton
R. I. Hynes, University of Southampton
D. Steeghs, University of Southampton
K. S. Long, Space Telescope Science Institute
S. Araujo-Betancor, University of Southampton
T. R. Marsh, University of Southampton


We report the discovery of 15 s oscillations in ultraviolet observations of WZ Sagittae obtained with the Hubble Space Telescope approximately 1 month after the peak of the 2001 outburst. This is the earliest detection of oscillations in WZ Sge following an outburst and the first time that a signal near 15 s has been seen to be dominant. The oscillations are quite strong (amplitude ≲5%), but not particularly coherent. In one instance, the oscillation period changed by 0.7 s between successive observations separated by less than 1 hr. We have also found evidence for weaker signals with periods near 6.5 s in some of our data. We discuss the implications of our results for the models that have been proposed to account for the 28 s oscillations seen in quiescence. If the periods of the 15 s oscillations can be identified with the periods of revolution of material rotating about the white dwarf, the mass of the white dwarf must satisfy MWD > 0.71 M⊙. The corresponding limit for the 6.5 s signals is MWD > 1.03 M ⊙.