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We report the results of a Dark Energy Camera (DECam) optical follow-up ofthe gravitational wave (GW) event GW151226, discovered by the Advanced LIGOdetectors. Our observations cover 28.8 deg$^2$ of the localization region inthe $i$ 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 limitingmagnitudes 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 arapidly declining optical counterpart with $\gtrsim 3\sigma$ significancesteady decline between the first and final observations. We recover foursources that pass our selection criteria, of which three are cataloged AGN. Thefourth source is offset by $5.8$ arcsec from the center of a galaxy at adistance of 187 Mpc, exhibits a rapid decline by $0.5$ mag over $4$ days, andhas a red color of $i-z\approx 0.3$ mag. These properties roughly match theexpectations 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 itslong-term behavior, PS15cdi is likely a Type IIP supernova that transitionedout of its plateau phase during our observations, mimicking a kilonova-likebehavior. We comment on the implications of this detection for contamination infuture optical follow-up observations.

A DECam Search for an Optical Counterpart to the LIGO Gravitational Wave Event GW151226