We present simultaneous UV-G-R-I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf 2MASSW J1707183+643933 with an amplitude in the UV of at least 6 magnitudes. This is one of the strongest detections ever of an optical flare on an M star and one of the first in an ultracool dwarf (UCD, spectral types later than about M7). Four intermediate strength flares (Delta m_UV < 4 mag) were found in this and three other targets. For the whole sample we deduce a flare probability of 0.013 (rate of 0.022/hr), and 0.049 (0.090/hr) for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong Halpha line emission. We also confirm the previously found rotation period for 2M1707+64 (Rockenfeller, Bailer-Jones & Mundt (2006)) and determine it more precisely to be 3.619 +/- 0.015 hr.
Monthly Notices of the Royal Astronomical Society, in press, 2006
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[arXiv]