The Mouse That Roared: A Superflare from the dMe Flare Star EV Lac Detected by Swift and Konus-Wind

• Published in: The Astrophysical journal Vol. 721; no. 1; pp. 785 - 801
• Main Authors: Osten, Rachel A, Godet, Olivier, Drake, Stephen, Tueller, Jack, Cummings, Jay, Krimm, Hans, Pye, John, Pal'shin, Valentin, Golenetskii, Sergei, Reale, Fabio, Oates, Samantha R, Page, Mat J, Melandri, Andrea
• Format: Journal Article
• Language: English
• Published: United States IOP Publishing 20.09.2010
• Subjects: ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERES; BOLOMETERS; ELECTROMAGNETIC RADIATION; ELEMENTS; EMISSION; FLUORESCENCE; HARD X RADIATION; IONIZATION; IONIZING RADIATIONS; IRON; LUMINESCENCE; LUMINOSITY; MEASURING INSTRUMENTS; METALS; OPTICAL PROPERTIES; PHOTON EMISSION; PHYSICAL PROPERTIES; RADIATIONS; SATELLITES; SIMULATION; SOFT X RADIATION; STARS; STELLAR ACTIVITY; STELLAR ATMOSPHERES; STELLAR CORONAE; STELLAR FLARES; TELESCOPES; TRANSITION ELEMENTS; X RADIATION
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/721/1/785

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{sub X} from 0.3 to 100 keV of 5.3 x 10{sup -8} erg cm{sup -2} s{sup -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{sub bol}) during the early stages of the flare, with a peak estimated L{sub X}/L{sub bol} {approx} 3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/R{sub *} = 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{sub *} = 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 {approx}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.