A Hubble Space Telescope STIS Observation of VW Hydri at the Exact Far-Ultraviolet Onset of an Outburst

Abstract

We present an analysis of Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph data of VW Hyi that we acquired 14 days after a superoutburst. At the time of our observation, the system appears to be going into outburst with the longest wavelengths increasing in flux by a factor of 5 while the shortest wavelengths increase by only a factor of 2. Using the distance of 65 pc, a system inclination angle of 60°, and a white dwarf mass of 0.86 M_solar, we carried out model fits involving a white dwarf by itself; an optically thick accretion disk by itself; a composite model using an optically thick accretion disk and a white dwarf; a two-temperature white dwarf model with a cooler, more slowly rotating photosphere and a hotter, rapidly rotating accretion belt; and a composite model involving a white dwarf and a rapidly rotating cooler disk ring heated up to a ``low'' temperature of 13-14,000 K. This component of temperature stays fairly constant throughout the HST observations, while the area of the disk ring increases by a factor of 12. We see evidence of a delay in the UV emission consistent with the outburst beginning outside of a disk truncation radius.

Based on observations made with the Hubble Space Telescope. HST is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.