EUVE All-Sky Survey Observations of the Dwarf Nova VW Hydri

The dwarf nova VW Hyi was observed from 1992 November 1 to 18 and from November 20 to 23 by the Extreme Ultraviolet Explorer (EUVE) satellite during its all-sky survey. The total time on source was 22 kiloseconds. During the first part of the scan, VW Hyi was in quiescence following a superoutburst and was not detected by EUVE above background. However, the source went into a narrow outburst on November 13.8 U.T., peaked at V ~ 10 on November 14.8 U.T., started to fade on November 15.9 U.T., and returned to quiescence on November 17.8 U.T. In contrast, the source did not turn on in the EUV until November 15.0 U.T., did not peak until November 15.3 U.T., and had returned to quiescence by November 16.3 U.T. Accounting for the different sensitivities in the two wavebands, we conclude that the EUV outburst was delayed relative to the optical outburst by ~ 0.5 day and that the EUV outburst was narrower than the optical outburst by ~ 1 day. During the peak of the EUV outburst, the source was detected at the 3sigma level in the Lex/B (50--180 Angstroms ) scanner with a count rate of ~ 0.03 s(-1) and at the 4sigma level in the Al/Ti/C (160--240 Angstroms ) scanner with a count rate of ~ 0.09 s(-1) . That the Al/Ti/C count rate is significantly higher than the Lex/B count rate, whereas the effective area of the Al/Ti/C filter is smaller than that of the Lex/B filter, strongly indicates that the source was very soft when it was on. We use the count rates in these two filters to constrain the temperature and luminosity of the source of the shortest wavelength radiation in VW Hyi: the inner disk and the boundary layer between the disk and the surface of the white dwarf.