Very Bright, Very Hot and Very Long: Swift Observations of the DG CVn "Superflare" of April 23rd, 2014
Abstract
On April 23rd this year, one of the 2 stars in the close visual binary dM4e system DG CVn flared to a level bright enough 300 milliCrab in the 15-150 keV band) that it triggered the Swift Burst Alert Telescope. Two minutes later, after Swift had slewed to the direction of this source, the Swift X-ray Telescope (XRT) and the Ultraviolet Optical Telescope (UVOT) commenced observing this flare. These observations continued (intermittently) for about 20 days and yielded a fascinating case history of this colossal event, the decay of which took more than a week in the UV and soft X-ray regions, and included several smaller superimposed secondary flares. The peak 0.3-10 keV luminosity observed by the XRT of 1.9e32 erg/s at the 18 pc distance of this system is 1.5 times the 'normal' combined systemic bolometric luminosity of 1.3e32 erg/s, making this event a super-bolometric flare similar to the 2008 flare of EV Lac (also detected by Swift). The BAT and XRT spectra of this flare in the first 6 minutes indicate that the emission was dominated by very hot (>>10 keV) plasma and/or a non-thermal power-law emission. This flare is arguably the longest, most X-ray luminous and hottest flare ever seen for an M dwarf in the solar neighborhood, and is reminiscent of the 9 days long flare of the RS CVn binary CF Tuc detected by ROSAT. We discuss how these exceptional characteristics may be related to the known properties of this system, specifically to its youth (30 Myr) and rapid rotation (55 km/s).
- Publication:
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AAS/High Energy Astrophysics Division #14
- Pub Date:
- August 2014
- Bibcode:
- 2014HEAD...1440406D