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

Abstract

On 2014 April 23, 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∼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×10 35 and 9×10 35 erg, and optical flare energies at E V of 2.8×10 34 and 5.2×10 34 erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

Kepler Data on KIC 7341653: A Nearby M Dwarf with Monster Flares and a Phase-coherent Variability

“…These properties put the mid-type M dwarf in the category of extremely active UV Cet-type stars. The energy released during the largest flare is similar to the event detected by Osten et al (2016) for DG CVn in the optical bandpass.…”

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

“…The Hγ/C4170 value is higher than in the observations but still in general agreement. Apparently, the same two-component multithread model in the decay phase of an E∼10 36 erg secondary flare event (F2; Osten et al 2016) during the DG CVn Superflare can also adequately explain the Balmer line and continuum flux properties in the decay phase (S#24) of the secondary flare events (e.g., at t 1.1 hr = in Figure 8) in the YZ CMi Megaflare. For a comparison of the continuum flux properties in the DG CVn Superflare and in the YZ CMi Megaflare, we refer the reader to Osten et al (2016).…”

“…Following the analysis of the DG CVn superflare presented in Osten et al (2016), we assume X X 25 decay kernels = and X 0 hotspot = for S#24 and solve for X kernels using the flare-only, specific continuum flux centered at 4170 l = Å and 4785 l = Å observed at Earth. For S#113, we assume X X 25 decay kernels = and find that X X 2 hotspot kernels = by fitting to the Balmer jump ratio (C3615/C4170=1.5) in the observation; then we solve for X kernels using the flare-only, specific continuum flux centered at 4170 l = Å and 4785 l = Å observed at Earth.…”

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…Table 1 provides comparison points for solar flares, and in Table 8 we note the GOES classification and other parameters for well-known solar and stellar flares to give context for the GJ −2 ) would make this an X2300-class flare. Osten et al (2016) found that the superflare observed from the young M dwarf binary system DG CVn was equivalent to an X600,000-class flare (60 W m −2 at 1 au, or 6000 W m −2 at 0.1 au, the approximate HZ distance). We note, however, that there is evidence that SXR-proton scaling relations should break down for such large events (>X10-class; Hudson 2007; Drake et al 2013).…”