Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen

Abstract We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted Kiso Schmidt telescope. detect 22 from M3–M5 dwarfs with rise time of 5 s ? trise 100 and an amplitude 0.5 ?F/F? 20. The flare light-curves mostly show steeper rises shallower decays than those obtained Kepler one-minute cadence data tend to have flat peak structures. Assuming blackbody spectrum temperature 9000–15000 K, luminosities energies are estimated be 1029 erg s?1 Lpeak 1031 Eflare 1034 erg, which constitutes bright end fast optical M dwarfs. confirm that more $90\%$ host stars detected magnetically active based their H?-emission-line intensities by LAMOST. An occurrence rate is ?0.7 per day star, indicating they common in argue can explained chromospheric compression model: broadly consistent Alfvén transit magnetic loop length scale lloop ? 104 km field strength 1000 gauss, while decay likely determined radiative cooling compressed chromosphere down near photosphere ? 10000 K. These could major contamination source future search transients unknown types.