A Challenge to Identify an Optical Counterpart of the Gravitational Wave Event GW151226 with Hyper Suprime-Cam

TL;DR

This study used the Subaru Hyper Suprime-Cam to search for optical counterparts to the GW151226 gravitational wave event, identifying potential kilonova candidates based on their optical properties and temporal behavior.

Contribution

It provides a detailed analysis of optical imaging data for GW151226, demonstrating methods to discriminate kilonovae from other transients and assessing the feasibility of future follow-up observations.

Findings

1744 candidates detected in the survey

Most candidates are likely flare stars in the galactic plane

Kilonova detection feasible within 200 Mpc with improved localization

Abstract

We present the results of the detailed analysis of an optical imaging survey conducted using the Subaru / Hyper Suprime-Cam (HSC), which aims to identify an optical counterpart to the gravitational wave event GW151226. In half a night, the $i$- and $z$-band imaging survey by HSC covers 63.5deg$^2$ of the error region, which contains about 7\% of the LIGO localization probability, and the same field is observed in three different epochs. The detectable magnitude of the candidates in a differenced image is evaluated as $i \sim 23.2$ mag for the requirement of at least two 5$\sigma$ detections, and 1744 candidates are discovered. Assuming a kilonova as an optical counterpart, we compared 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 galactic plane, $|b|<5^{\circ}$, supports this interpretation. We also checked 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 $\sim50{\rm deg}^2$) is available, kilonova searches of up to approximately $200$ Mpc will become feasible by conducting immediate follow-up observations with an interval of 3--6 days.