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

Abstract

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}$ cm${}^2$, 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$\sim$10${}^4$K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4$\times$10${}^{35}$ and 9$\times$10${}^{35}$ erg, and optical flare energies 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 updated modelling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

48 pages, 9 figures, accepted for publication in the Astrophysical Journal