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 fX from 0.3 to 100 keV of 5.3 × 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 ~ 3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/R sstarf = 0.37 ± 0.07. The soft X-ray spectrum of the flare reveals evidence of iron Kα emission at 6.4 keV. We model the Kα emission as fluorescence from the hot flare source irradiating the photospheric iron, and derive loop heights of h/R sstarf = 0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The Kα emission feature shows variability on timescales of ~200 s which is difficult to interpret using the pure fluorescence hypothesis. We examine Kα 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 Kα flux and the lack of observed nonthermal emission in the 20-50 keV spectral region.