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

Abstract

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 0.3-100 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 ⋆ = 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 ⋆ =0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The Kα emission feature shows variability on time scales 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.

Two fast X-ray transients in archival Chandra data

“…Larger peak X-ray luminosities have been reported in superflares for earlier type M dwarfs (e.g. Osten et al 2010). This may be the largest X-ray flare detected in such a late type dwarf, although the distance and spectral type are uncertain.…”

A VERY BRIGHT, VERY HOT, AND VERY LONG FLARING EVENT FROM THE M DWARF BINARY SYSTEM DG CVn

“…As described in Osten et al (2010) applied to Swift data, the thermodynamic loop decay time can be expressed (Serio et al 1991 and T obs is the maximum best-fit temperature derived from single temperature fitting of the data. The ratio of the observed exponential light-curve decay time τ LC to the thermodynamic decay time τ th can be written as a function that depends on the slope ζ of the decay in the log(n e -T e ) plane (or equivalently, ( )  -T log plane) and other parameters…”

“…There is a large disparity between the extremes of solar and stellar flares: while the largest solar flares have radiated energies exceeding 10 32 erg, and maximum coronal temperatures of a few tens of MK (Sharykin et al 2015), large stellar flares can be 10 6 times more energetic, with coronal temperatures around 100 MK (Osten et al 2007) and large energetic releases up to 10 38 erg (Kuerster & Schmitt 1996;Osten et al 2007). A 2008 flare of the nearby 30-300 Myr old M dwarf flare star EV Lac (Osten et al 2010) had a lower limit on the energetic release of 6×10 34 erg. Caramazza et al (2007) found X-ray flares on very young low-mass stars to range up to 2×10 35 erg, and Tsuboi et al (2014) found flares from active binary systems to range up to 10 38 erg.…”

RecoveringSwift-XRT energy resolution through CCD charge trap mapping

“…Osten et al (2010) modelled the WT mode spectrum with a two-temperature APEC model (Smith et al 2001) and reported the additional detection of a fluorescent iron line at 6.4 keV during the initial flare decay. An enhancement of the visibility of the emission lines is expected when trap corrections are applied, as in the case of the lines in the Cas A and Tycho SNRs.…”

“…A partial covering absorber was studied in the soft Seyfert 1 WPVS 007 (Grupe et al 2008). Furthermore, X-ray spectroscopy is key in the physical investigation of X-ray binaries (Romano et al 2011), flaring stars (EV Lac, Osten et al 2010) and comets (Comet 9P/Temple, Willingale et al 2006). …”