A Challenge To Identify An Optical Counterpart Of The Gravitational Wave Event Gw151226 With Hyper Suprime-Cam(Dagger)

We present the results of detailed analysis of an optical imaging survey conducted using the Subaru/Hyper Suprime-Cam (HSC) that aimed to identify an optical counterpart to the gravitational wave event GW151226. In half a night, the i- and z-band imaging survey by HSC covered 63.5 deg(2) of the error region, which contains about 7% of the LIGO localization probability, and the same field was observed in three different epochs. The detectable magnitude of the candidates in a differenced image is evaluated as i similar to 23.2 mag for the requirement of at least two 5 sigma detections, and 1744 candidates are discovered. Assuming a kilonova as an optical counterpart, we compare the optical properties of the candidates with model predictions. A red and rapidly declining light curve condition enables the discrimination of a kilonova from other transients, and a small number of candidates satisfy this condition. The presence of stellar-like counterparts in the reference frame suggests that the surviving candidates are likely to be flare stars. The fact that most of those candidates are in the galactic plane, vertical bar b vertical bar< 5 degrees, supports this interpretation. We also check whether the candidates are associated with the nearby GLADE galaxies, which reduces the number of contaminants even with a looser color cut. When a better probability map (with localization accuracy of similar to 50 deg(2)) is available, kilonova searches of up to approximately 200Mpc will become feasible by conducting immediate follow-up observations with an interval of 3-6 d.