  We present the first detection of photometric variability of a
spectroscopically-confirmed Y dwarf. The Infrared Array Camera on board the
Spitzer Space Telescope was used to obtain times series photometry at 3.6 and
4.5 microns over a twenty four hour period at two different epochs separated by
149 days. Variability is evident at 4.5 um in the first epoch and at 3.6 and
4.5 um in the second epoch which suggests that the underlying cause or causes
of this variability change on the timescales of months. The second-epoch [3.6]
and [4.5] light curves are nearly sinusoidal in form, in phase, have periods of
roughly 8.5 hours, and have semi-amplitudes of 3.5%. We find that a simple
geometric spot model with a single bright spot reproduces these observations
well. We also compare our measured semi-amplitudes of the second epoch light
curves to predictions of the static, one-dimensional, partly cloudy and hot
spot models of Morley and collaborators and find that neither set of models can
reproduce the observed [3.6] and[4.5] semi-amplitudes simultaneously. More
advanced two- or three-dimensional models that include time-dependent phenomena
like vertical mixing, cloud formation, and thermal relaxation are therefore
sorely needed in order to properly interpret our observations.
