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The extreme-ultraviolet continuum of a strong stellar flare

We present the serendipitous detection of an extreme-ultraviolet flare on EUVE J0613-3.9B. The flare showed over a 200-fold increase above the quiescent emission in the DS/Lexan 60-200 A wavelength band. Optical spectroscopy revealed that the event was associated with an active dM3.5e star. The EUVE spectra are dominated by emission lines formed at temperatures in excess of 107 K. The observation is unique as we have detected, for the first time, a strong Lyman continuum in the EUVE long-wavelength range (320-650 A). The flare in the continuum (T ~=20,000-30,000 K) was extremely short, lasting for less than 500 s, while in the DS (T ~=107 K) its duration was ~=28 ks. The total energy of the flare in the DS is ~3x10-34 ergs. We have made a fit to the continuum using semiempirical model atmospheres and derived the time-averaged temperature and density structures.

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