* Astronomy

Title: A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn
Author: Rachel A. Osten (Space Telescope Science Institute and Center for Astrophysical Sciences, Johns Hopkins University), Adam Kowalski (U. Md/GSFC and Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, and National Solar Observatory, University of Colorado Boulder), Stephen A. Drake (USRA/CRESST and NASA/GSFC), Hans Krimm (USRA/CRESST), Kim Page (University of Leicester), Kosmas Gazeas (University of Athens), Jamie Kennea (Penn State), Samantha Oates (Instituto de Astrofsica de Andaluc ía (IAA-CSIC)), Mathew Page (UCL), Enrique de Miguel (Universidad de Huelva, Spain), Rudolf Novák (Masaryk University, Czech Republic), Tomas Apeltauer (Brno University of Technology, Czech Republic), Neil Gehrels (NASA/GSFC)

On April 23, 2014, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG~CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T_X of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >10^20 cm², and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T~10^4K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4 x 10^35 and 9 x 10^35 erg, and optical flare energies at EV of 2.8 x 10^34 and 5.2 x 10^34 erg, respectively. The results presented here should be integrated into updated modelling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

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RA 13 31 46.61, Dec +29 16 36.62