EUVE Observations of U Geminorum in Outburst

We have observed U Gem during the peak and declining phases of a wide outburst in 1993 December with the Extreme Ultraviolet Explorer (EUVE) satellite. At peak, U Gem was one of the brightest EUV sources in the sky. The spectrum of the source is complex. Fitted to a blackbody spectrum, the apparent temperature at peak is ∼140,000 K, the luminosity is ∼4 x 10³⁴(D/90 pc)² ergs s^-1, and the minimum size of the emitting region is comparable to that of the white dwarf (WD). If the EUV emission arises primarily from the boundary layer, then the boundary layer luminosity in U Gem is comparable to the disk luminosity. The EUV source is partially eclipsed at orbital phases 0.6-0.8. The eclipse spectrum, which we associate with a wind emerging from the vicinity of the white dwarf, is dominated by emission features. The identification of these emission features with transitions expected in a relatively cool (T < 160,000 K), photoionized plasma helps to resolve a controversy concerning the ionization state of winds of dwarf novae. The EUV lines arise from the dominant ionization states of the wind, and their strengths suggest that the wind mass-loss rate, at least in U Gem, is a substantial fraction of the WD accretion rate.