THE MOUSE THAT ROARED: A SUPERFLARE FROM THE dMe FLARE STAR EV LAC DETECTED BY SWIFT AND KONUS-WIND

We report on a large stellar flare from the nearby dMe flare star EV Lac observed by the Swift and Konus-Wind satellites and the Liverpool Telescope. It is the first large stellar flare from a dMe flare star to result in a Swift trigger based on its hard X-ray intensity. Its peak f(X) from 0.3 to 100 keV of 5.3 x 10(-8) erg cm(-2) s(-1) is nearly 7000 times larger than the star's quiescent coronal flux, and the change in magnitude in the white filter is >= 4.7. This flare also caused a transient increase in EV Lac's bolometric luminosity (L-bol) during the early stages of the flare, with a peak estimated L-X/L-bol similar to 3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/R-star = 0.37 +/- 0.07. The soft X-ray spectrum of the flare reveals evidence of iron K alpha emission at 6.4 keV. We model the K alpha emission as fluorescence from the hot flare source irradiating the photospheric iron, and derive loop heights of h/R-star = 0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The K alpha emission feature shows variability on timescales of similar to 200 s which is difficult to interpret using the pure fluorescence hypothesis. We examine K alpha emission produced by collisional ionization from accelerated particles, and find parameter values for the spectrum of accelerated particles which can accommodate the increased amount of K alpha flux and the lack of observed nonthermal emission in the 20-50 keV spectral region.