EUVE Observations of a Giant Flare from UX ARIETIS

The active RSCVn-type binary star UX Arietis (HD 21242) was fortuitously caught in a high flaring state when the Extreme Ultraviolet Explorer satellite began observations on 19 Nov. 1995 (UT 10:45). The count rate in the Deep Survey (DS) instrument that samples a band comparable to the Short Wavelength spectrograph (70 Angstroms to 140 Angstroms) was a factor of ten higher than measured during a previous observing sequence on 7--10 Nov. 1995. This event is not only the brightest RSCVn flare observed by EUVE, but also marks the largest relative increase in source brightness for a RSCVn star ever measured by EUVE. Thanks to the EUVE Target of Opportunity program, our pre-planned observing time was extended to 25 November 1995 (UT 11:00), enabling the decay of the flare to be monitored over almost the complete orbital period of 6.44 days. During the decay, two other weaker flares occurred as detected by the DS instrument. UX Ari had not returned to its pre-flare emission level by the end of the observations, but the DS count rate remained a factor of 3 above its quiescent state. Our study of previous euv spectra of UX Ari shows an emission measure distribution dominated by a narrow enhancement (a ``bump'') at high temperatures ( ~ 10(7) K) similar to that found in other short-period RSCVn systems. The spectra (70 Angstroms -- 400 Angstroms) of the flaring event are dominated by emission lines of He II and highly ionized iron (in particular Fe XX, XXIII, and XXIV) which have substantially increased flux as compared to the quiescent state. Emission measures and densities can be derived from spectra obtained at various times in order to follow the decay of the coronal plasma.