We report on the results of fifteen 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 H \alpha emission .
A preliminary trigonometric parallax shows that W1906+40 is at a distance of 16.35 ^ { +0.36 } _ { -0.34 } 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 hours and an amplitude of 1.5 % , with a consistent phase throughout the year .
We infer a radius of 0.92 \pm 0.07 R _ { J } and \sin i > 0.57 from the observed period , luminosity ( 10 ^ { -3.67 \pm 0.03 } L _ { \odot } ) , effective temperature ( 2300 \pm 75 K ) , and v \sin i ( 11.2 \pm 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 \gtrsim 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 can not 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 \sim 1.6 \times 10 ^ { 32 } ergs in white light emission .
Using the Kepler data , we identify similar flares and estimate that white light flares with optical/ultraviolet energies of 10 ^ { 31 } ergs or more occur on W1906+40 as often as 1-2 times per month .