Multiband photometric detection of a huge flare on the M9 dwarf 2MASSW J1707183+643933

We present simultaneous UV, G, R and I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf with an amplitude in the UV of at least 6 mag. This is one of the strongest detections ever of an optical flare in an M star and one of the first in an ultracool dwarf (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 h(-1)), and 0.049 (0.090 h(-1)) for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong H alpha line emission. We also confirm our previously found rotation period for 2M1707+64 and determine it more precisely to be 3.619 +/- 0.015 h.