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On April 23, 2014, the Swift satellite responded to a hard X-ray transientdetected by its Burst Alert Telescope, which turned out to be a stellar flarefrom 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 0.3-100keV bandpass by either a single very high temperature plasma or a nonthermalthick-target bremsstrahlung model, and we rule out the nonthermal model basedon energetic grounds. The temperatures were the highest seen spectroscopicallyin a stellar flare, at T$_{X}$ of 290 MK. The first event was followed by acomparably energetic event almost a day later. We constrain the photosphericarea involved in each of the two flares to be $>$10$^{20}$ cm$^{2}$, and findevidence from flux ratios in the second event of contributions to the whitelight flare emission in addition to the usual hot, T$\sim$10$^{4}$K blackbodyemission seen in the impulsive phase of flares. The radiated energy in X-raysand white light reveal these events to be the two most energetic X-ray flaresobserved from an M dwarf, with X-ray radiated energies in the 0.3-10 keVbandpass of 4$\times$10$^{35}$ and 9$\times$10$^{35}$ erg, and optical flareenergies at E$_{V}$ of 2.8$\times$10$^{34}$ and 5.2$\times$10$^{34}$ erg,respectively. The results presented here should be integrated into updatedmodelling of the astrophysical impact of large stellar flares on close-inexoplanetary atmospheres.

A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn