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 $\sim 50$ 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 $\Delta V\approx -7.1$. The total equivalent duration of the flare is 15.4 hr. We estimate the total bolometric energy of the flare was $4\times 10^{33}$ erg, more powerful than the previously reported Kepler white light flares for the L1 dwarf WISEP J190648.47+401106.8, but weaker than the $\Delta V=-11$ 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 $\sim 70$ times the stellar photosphere, or $\Delta V\approx -8$. This flare is comparable to the most powerful Kepler flares observed on the active M4 dwarf GJ 1243.

Accepted to ApJ

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• A Study of Flares in the Ultra-Cool Regime from SPECULOOS-South
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• Hubble Space Telescope astrometry of the closest brown dwarf binary system -- I. Overview and improved orbit
• Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey
• Minute-cadence Observations of the LAMOST Fields with the TMTS: III. Statistic Study of the Flare Stars from the First Two Years
• The Puzzling Story of Flare Inactive Ultra Fast Rotating M dwarfs. II. Searching for radial velocity variations
• A huge-amplitude white-light superflare on a L0 brown dwarf discovered by GWAC survey
• Fast Photometric Variability of Very Low Mass Stars in IC 348: Detection of Superflare in an M-dwarf