DETECTION OF A STELLAR FLARE AT EXTREME ULTRAVIOLET WAVELENGTHS

THE transition region between stellar chromospheres and coronae contains plasma at temperatures of 10(5)-10(6) K, radiating predominantly at extreme ultraviolet (EUV) wavelengths (60-1,000 angstrom; 0.012-0.2 keV). To understand the energy transport processes that maintain the corona, radiative losses from this region must be studied, particularly during the dramatic energy release that occurs in a stellar flare. During the all-sky survey conducted by the Rosat Wide Field Camera 1, we monitored the binary flare star system BY Draconis, with coverage by the International Ultraviolet Explorer satellite far ultraviolet and optical observations and by the Rosat X-ray telescope 2 for part of the time. We detected a stellar flare in all four wavebands. This is the first unambiguous extreme ultraviolet detection of a flare, and one of the widest simultaneous wavelength-range coverages obtained. The peak luminosity and total energy of this flare, in the photon energy range 0.08-0.18 keV, are comparable with the values obtained for a number of flares integrated over a larger energy range (0.05-2.0 keV) by Exosat satellite observations in 1983-86. We conclude that radiation in the extreme ultraviolet carries away a substantial fraction of the total flare energy.