The All-Sky Automated Survey for Supernovae (ASAS-SN) is the only project in existence to scan the entire sky in optical light every ∼day, reaching a depth of g∼18 mag. Over the course of its first four years of transient alerts (2013-2016), ASAS-SN observed 53 events classified as likely M dwarf flares. We present follow-up photometry and spectroscopy of all 53 candidates, confirming flare events on 47 M dwarfs, one K dwarf, and one L dwarf. The remaining four objects include a previously identified TT Tauri star, a young star with outbursts, and two objects too faint to confirm. A detailed examination of the 49 flare star light curves revealed an additional six flares on five stars, resulting in a total of 55 flares on 49 objects ranging in V-band contrast from ΔV=−1 to −10.2 mags. Using an empirical flare model to estimate the unobserved portions of the flare light curve, we obtain lower limits on the V-band energy emitted during each flare, spanning log(EV/ergs)=32 to 35, which are among the most energetic flares detected on M dwarfs. The ASAS-SN M-dwarf flare stars show a higher fraction of Hα emission as well as stronger Hα emission compared to M dwarfs selected without reference to activity, consistent with belonging to a population of more magnetically active stars. We also examined the distribution of tangential velocities, finding that the ASAS-SN flaring M dwarfs are likely to be members of the thin disk and are neither particularly young nor old.
@article{arxiv.1809.04510,
title = {The largest M dwarfs flares from ASAS-SN},
author = {Sarah J. Schmidt and Benjamin J. Shappee and Jennifer L. van Saders and K. Z. Stanek and Jonathan S. Brown and C. S. Kochanek and Subo Dong and Maria R. Drout and Stephen Frank and Thomas W. -S. Holoien and Sean Johnson and Barry F. Madore and Jose L. Prieto and Mark Seibert and Marja K. Seidel and Gregory V. A. Simonian},
journal= {arXiv preprint arXiv:1809.04510},
year = {2019}
}