Kepler monitoring of an L dwarf I. The photometric period and white light flares

We report on the results of 15 months of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 (W1906+40) with the Kepler mission. Supporting observations with the Karl G. Jansky Very Large Array and Gemini North Telescope reveal that the L dwarf is magnetically active, with quiescent radio and variable Hα emission. A preliminary trigonometric parallax shows that W1906+40 is at a distance of 16.35_−0.34^+0.36 pc, and all observations are consistent with W1906+40 being an old disk star just above the hydrogen-burning limit. The star shows photometric variability with a period of 8.9 hr and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of 0.92 ± 0.07R_J and sin i > 0.57 from the observed period, luminosity (10^{–3.67 ± 0.03} L _☉), effective temperature (2300 ± 75 K), and vsin i (11.2 ± 2.2 km s^–1). The light curve may be modeled with a single large, high latitude dark spot. Unlike many L-type brown dwarfs, there is no evidence of other variations at the ≳ 2% level, either non-periodic or transient periodic, that mask the underlying rotation period. We suggest that the long-lived surface features may be due to starspots, but the possibility of cloud variations cannot be ruled out without further multi-wavelength observations. During the Gemini spectroscopy, we observed the most powerful flare ever seen on an L dwarf, with an estimated energy of ∼1.6 × 10³² erg in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of 10³¹ erg or more occur on W1906+40 as often as 1-2 times per month.