A major optical flare on the recently discovered X - ray active dMe star G 102 - 21

During the course of UBV photoelectric measurements made at the Catania Astrophysical Observatory we have observed an unusually intense optical flare on the nearby M dwarf G 102-21, the optical counterpart of a strong X-ray source recently detected by ROSAT. The amplitude of the magnitude increase (∆U ∼ 7.3 mag) and the total energy release in the UBV bands (E UBV ∼ (1.4 ± 0.3) × 10 35 erg) put this event among the strongest flares ever detected on UV Ceti-type stars. The U-B and B-V colours of the flare emission did not significantly change during most of the flare development. While the B-V is quite a typical colour for stellar flares, the U-B is much bluer when compared with typical values given in literature. This unusually strong ultraviolet component of the flare emission, together with the very fast flare decay, suggest a possible flare site close to the limb. We find that the observed amount of flare energy could be explained by the fast release of energy stored in a magnetic filament located in between the two G 102-21 stellar components, with a surface magnetic field ≥3600 G. The characteristics of the quiescent optical emission are also discussed.