We present time-series optical spectroscopy of two stellar flares with complex multi-peaked light curves: a 2.1_-.3_^+.4^x10^34^erg flare from an M8 ultra-cool dwarf, and a 1.5_-.2_^+.2^x10^35^erg superflare from an M4 field star. Both flares were discovered by the Evryscope Fast Transient Engine and observed with the Southern Astrophysical Research 4.1m telescope ~10min after their initial peak. We model the blackbody temperature of the flares as a function of time, finding that the characteristic temperature reaches ~13,000K during the secondary peaks of both events and is likely higher during the initial, unobserved impulsive phase. Both nonthermal emission from the near-UV Balmer continuum and an evolving red continuum of uncertain origin are observed in both flares; we compare the relative contributions of the continua, line emission, and blackbody component as the flare evolves, and find that the secondary peak of the M4 superflare immediately follows inversion of the relative contributions of the Balmer and red continua. We additionally present a model that constrains the evolution of the electron density of the stellar chromosphere during the flare, based on the time evolution of the observed red continuum excess, which we interpret as components of the hydrogen recombination continuum. Our observations of a 10.5mag flare from an M8 dwarf, as well as a similar, unusually energetic event from an M4 dwarf, are comparable in intensity to the most energetic stellar flares reported in the literature to date.

Cone search capability for table J/AJ/169/302/table1 (EFTE Flare discovery coordinates, number of significant detections in Evryscope, peak magnitudes, and time stamps for detection and alert generation)