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Swift Detection of a Superflare from EV Lac

ATel #1499; R. Osten (GSFC/UMD), S. Drake (GSFC/USRA), O. Godet (Leicester), J. Pye (Leicester), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), V. Pal'shin (Ioffe Inst.), S. Golenetskii (Ioffe Inst.), J. Tueller (GSFC), H. Krimm (GSFC/USRA)
on 30 Apr 2008; 13:59 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Star

Referred to by ATel #: 1718, 1787, 6121

The Swift team reports the detection of a superflare from the dMe flare star EV Lac. The Burst Alert Telescope (BAT) triggered on EV Lac during a pre-planned slew at 2008-04-25T05:13:57 = T0(BAT), and resulted in an automatic trigger slew to the source. The hard X-ray source was already declining in flux as it entered the BAT field of view during the pre-planned slew, with a peak intensity in the BAT 15-50 keV band of around 660 mCrab, or 0.15 counts/cm2/s. The timescale for significant hard X-ray emission seen with BAT was short for a large stellar flare, lasting less than about 900 seconds. Konus-Wind reports a bump in the soft energy range (20-70 keV) seen only in the S2 detector which observes the north ecliptic hemisphere, starting at ~T0(BAT)-100 seconds, which likely indicates the beginning of the superflare, and a possible tail until ~T0+1000 seconds. There are signs of a precursor event at ~T0(BAT)-700 seconds. When the XRT started observing at T0(BAT)+165.6 seconds the soft X-ray rate was ~1000 counts/s and then decayed rapidly, reaching a count rate of ~2 counts/s roughly 10,000 seconds after the trigger. Preliminary analysis of the XRT data from T0+170s to T0+6100 s shows a 0.3-10 keV average flux of 7e-9 ergs/cm2/s, with a 2T MEKAL fit giving temperatures of 0.8 and 5 keV. The true peak XRT flux, based on the 1000 XRT counts/s at the XRT observation start and a 5 keV temperature plasma, is in excess of 3e-8 erg/cm2/s, corresponding to > 1.7 times the star's bolometric luminosity of 5.25e31 erg/s at the Hipparcos distance of 5 pc. The UVOT went into a safe mode because this star is too bright optically.

EV Lac was also observed to undergo a large stellar flare in 1998 caught by the ASCA satellite (Favata et al. 2000 A&A 353, 987) but the flare detected by Swift appears to be more than 7 times larger in the soft X-ray energy band. In comparison with the only other stellar flare on which Swift has triggered, that on II Peg described by Osten et al. (2007 ApJ 654,1052), the flare on EV Lac is about a factor of 3 higher flux at hard X-ray energies. The BAT transient monitor has observed three smaller (3-4 sigma) enhancements from EV Lac, on 6 June 2006, 20 May 2007, and 22 April 2008, in the range 130-260 mCrab (15-50 keV), which were not strong enough to trigger slews. In consideration of the large amount of soft and hard X-ray flux received at Earth from this flare on EV Lac, this may be the brightest stellar X-ray flare yet observed.