K2 Ultracool Dwarfs Survey. I. Photometry of an L Dwarf Superflare

Abstract

We report on K2 Campaign 8 measurements of a huge white light flare on the L1 dwarf SDSSp J005406.55-003101.8 (EPIC 220186653). The source is a typical L1 dwarf at a distance of ∼ 50 pc, probably an old hydrogen-burning star rather than a young brown dwarf. In the long (30-minute) cadence photometry, the flare peak is 21 times the flux of the stellar photosphere in the broad optical Kepler filter, which we estimate corresponds to ∆V ≈ −7.1. The total equivalent duration of the flare is 15.4 hr. We estimate the total bolometric energy of the flare was 4 × 1033 erg, more powerful than the previously reported Kepler white light flares for the L1 dwarf WISEP J190648.47+401106.8, but weaker than the ∆V = −11 L0 dwarf superflare ASASSN-16ae. The initial (impulsive) cooling phase is too rapid to resolve with our 30-minute cadence data, but after one hour the gradual cooling phase has an exponential time constant of 1.8 hours. We use template fitting to estimate that the full-time-width-at-half-amplitude of the light curve is < 10 minutes and that the true flare maximum reached ∼ 70 times the stellar photosphere, or ∆V ≈ −8. This flare is comparable to the most powerful Kepler flares observed on the active M4 dwarf GJ 1243.