Finding the Largest Flares on Ultracool Dwarfs with ASAS-SN

Quiescent chromospheric activity, as measured through Halpha emission, is ubiquitous on ultracool (late-M and early-L) dwarfs, but the rate of white-light flares on these objects is still under investigation. Recent work with Kepler and K2 has revealed that flares occur less frequently than on more massive M dwarfs, but the strongest flares are sufficiently rare that they are unlikely to be observed in the 90 day observational windows. The All Sky Automated Search for Supernovae (ASAS-SN) survey scans the entire sky once every two days in V band down to V> 17. In addition to discovering hundreds of Supernovae, the ASAS-SN survey has also observed hundreds of stellar flares, including two particularly dramatic flares in the ultracool regime; a ΔV ~ -9 on an M8 dwarf, and a ΔV ~ -10 flare on an L1 dwarf. Both flares radiated ~10³⁴ ergs in the V-band, placing them among the strongest observed white-light flares. While flares this strong are expected to occur less than once per year on individual ultracool dwarfs, the all-sky coverage of ASAS-SN presents a unique opportunity to detect strong flares (ΔV < -5) on all ultracool dwarfs within ~100pc. We discuss the two most dramatic ASAS-SN flares and present our initial constraints on the rate of large flares on ultracool dwarfs.