Kepler Monitoring of an L Dwarf II. Clouds with Multiyear Lifetimes

We present Kepler, Spitzer Space Telescope, Gemini-North, MMT, and Kitt Peak observations of the L1 dwarf WISEP J190648.47+401106.8. We find that the Kepler optical light curve is consistent in phase and amplitude over nearly two years of monitoring, with a peak-to-peak amplitude of 1.4%. Spitzer Infrared Array Camera 3.6 mu m observations are in phase with Kepler, with similar light curve shapes and peak-to-peak amplitudes of 1.1%, but at 4.5 mu m, the variability has an amplitude of <0.1%. Chromospheric Ha emission is variable but not synced with the stable Kepler light curve. A single dark spot can reproduce the light curve but is not a unique solution. An inhomogeneous cloud deck, specifically a region of thick cloud cover, can explain the multi-wavelength data of this ultracool dwarf and need not be coupled with the asynchronous magnetic emission variations. The long life of the cloud is in contrast with weather changes seen in cooler brown dwarfs on the timescale of hours and days.