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

TL;DR

This study used the DECam to follow up on the LIGO GW151226 event, searching for optical counterparts, and identified a transient that resembled a kilonova but was later confirmed unrelated, highlighting contamination challenges.

Contribution

First DECam follow-up of GW151226 covering 28.8 deg$^2$, analyzing transient candidates, and discussing contamination issues in optical counterpart searches.

Findings

Detected a transient with kilonova-like features that was unrelated to GW151226.

Most candidates were known AGN or supernovae, illustrating contamination challenges.

Provided insights into the rate and nature of false positives in optical follow-up.

Abstract

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.