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Swift detection of flares from V711 Tau

ATEL # 956; H. Krimm (GSFC/USRA), A. Beardmore (U. Leicester), S. Drake (GSFC/USRA), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), G. Sato (GSFC/ISAS), R. Osten (GSFC/UMD), L. Barbier (GSFC), E. Fenimore (LANL), N. Gehrels (GSFC), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on 1 Dec 2006; 23:30 UT
Password Certification: Hans A. Krimm (

Subjects: X-ray, Variables, Stars

The Swift team reports the detection of three bright hard X-ray flares from the RS CVn binary system V711 Tau (also known as HR 1099). The flares occurred on Nov 29, 2006 and were seen by the Burst Alert Telescope (BAT) on Swift in 64-second exposures starting at 04:30:39, 04:31:43, and 04:33:51 UTC. The intensity of the flares were 0.097, 0.071 and 0.088 +/- 0.013 counts/sec/cm^2, respectively, in the 15-50 keV band. These correspond to approximately 400 mCrab. Although the source has been in the BAT field of view at least four times since these outbursts, it has not been seen again in the BAT.

The source was also seen in the BAT on March 9, 2006 at 0.011 +/- 0.0017 counts/s/cm^2 or 50 mCrab. The V711 Tau system has a long history of big flares detected in the radio and X-ray.

A 2000-second Swift ToO observation was triggered on Nov. 30 at 20:45 UT (~40 hours after the flares). The source was clearly detected by the X-ray Telescope (XRT) in Windowed Timing mode at ~8 counts/sec. The spectrum is well fit with a 2 component mekal model and absorption. The best fit parameters are kT1 = 2.53 +/-0.15 keV, kT2 = 0.78 +/-0.03 keV, NH = (1.0 +/- 0.3) X 10^20 cm^-2, abundance = 0.21 +/- 0.04 solar. The relative normalization of the emission measures is 2.0 (hot/cool). The flux 0.5-10 keV is about 2.5 x 10^-10 erg/s/cm^2, which is enhanced compared to normal (0.7 x 10^-10 erg/s/cm^2) so the source may still have been in a long decay phase following the large event detected by BAT.

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R. E. Rutledge , Editor
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