Discovery of 15 Second Oscillations in [ITAL]Hubble Space Telescope[/ITAL] Observations of WZ Sagittae following the 2001 Outburst

Abstract

We report the discovery of 15 s oscillations in ultraviolet observationsof WZ Sagittae obtained with the Hubble Space Telescope approximately 1month after the peak of the 2001 outburst. This is the earliestdetection of oscillations in WZ Sge following an outburst and the firsttime that a signal near 15 s has been seen to be dominant. Theoscillations are quite strong (amplitude ~=5%), but not particularlycoherent. In one instance, the oscillation period changed by 0.7 sbetween successive observations separated by less than 1 hr. We havealso found evidence for weaker signals with periods near 6.5 s in someof our data. We discuss the implications of our results for the modelsthat have been proposed to account for the 28 s oscillations seen inquiescence. If the periods of the 15 s oscillations can be identifiedwith the periods of revolution of material rotating about the whitedwarf, the mass of the white dwarf must satisfy M_{WD}{\gt}0.71M_{solar}. The corresponding limit for the 6.5 s signals isM_{WD}{\gt}1.03 M_{solar}. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.