A DECam Search for an Optical Counterpart to the LIGO Gravitational Wave
  Event GW151226
  We report the results of a Dark Energy Camera (DECam) optical follow-up of
the gravitational wave (GW) event GW151226, discovered by the Advanced LIGO
detectors. Our observations cover 28.8 deg$^2$ of the localization region in
the $i$ and $z$ bands (containing 3% of the BAYESTAR localization probability),
starting 10 hours after the event was announced and spanning four epochs at
$2-24$ days after the GW detection. We achieve $5\sigma$ point-source limiting
magnitudes of $i\approx21.7$ and $z\approx21.5$, with a scatter of $0.4$ mag,
in our difference images. Given the two day delay, we search this area for a
rapidly declining optical counterpart with $\gtrsim 3\sigma$ significance
steady decline between the first and final observations. We recover four
sources that pass our selection criteria, of which three are cataloged AGN. The
fourth source is offset by $5.8$ arcsec from the center of a galaxy at a
distance of 187 Mpc, exhibits a rapid decline by $0.5$ mag over $4$ days, and
has a red color of $i-z\approx 0.3$ mag. These properties roughly match the
expectations for a kilonova. However, this source was detected several times,
starting $94$ days prior to GW151226, in the Pan-STARRS Survey for Transients
(dubbed as PS15cdi) and is therefore unrelated to the GW event. Given its
long-term behavior, PS15cdi is likely a Type IIP supernova that transitioned
out of its plateau phase during our observations, mimicking a kilonova-like
behavior. We comment on the implications of this detection for contamination in
future optical follow-up observations.
