A DECam Search for an Optical Counterpart to the LIGO Gravitational Wave Event GW151226
Creators: name: Cowperthwaite, P. S.
name: Berger, E.
name: Soares-Santos, M.
name: Annis, J.
name: Brout, D.
name: Brown, D. A.
name: Buckley-Geer, E.
name: Cenko, S. B.
name: Chen, H. Y.
name: Chornock, R.
name: Diehl, H. T.
name: Doctor, Z.
name: Drlica-Wagner, A.
name: Drout, M. R.
name: Farr, B.
name: Finley, D. A.
name: Foley, R. J.
name: Fong, W.
name: Fox, D. B.
name: Frieman, J.
name: Garcia-Bellido, J.
name: Gill, M. S. S.
name: Gruendl, R. A.
name: Herner, K.
name: Holz, D. E.
name: Kasen, D.
name: Kessler, R.
name: Lin, H.
name: Margutti, R.
name: Marriner, J.
name: Matheson, T.
name: Metzger, B. D.
name: Neilsen, E. H.
name: Quataert, E.
name: Rest, A.
name: Sako, M.
name: Scolnic, D.
name: Smith, N.
name: Sobreira, F.
name: Strampelli, G. M.
name: Villar, V. A.
name: Walker, A. R.
name: Wester, W.
name: Williams, P. K. G.
name: Yanny, B.
name: Abbott, T. M. C.
name: Abdalla, F. B.
name: Allam, S.
name: Armstrong, R.
name: Bechtol, K.
name: Benoit-Levy, A.
name: Bertin, E.
name: Brooks, D.
name: Burke, D. L.
name: Rosell, A. Carnero
name: Kind, M. Carrasco
name: Carretero, J.
name: Castander, F. J.
name: Cunha, C. E.
name: D'Andrea, C. B.
name: da Costa, L. N.
name: Desai, S.
name: Dietrich, J. P.
name: Evrard, A. E.
name: Neto, A. Fausti
name: Fosalba, P.
name: Gerdes, D. W.
name: Giannantonio, T.
name: Goldstein, D. A.
name: Gruen, D.
name: Gutierrez, G.
name: Honscheid, K.
name: James, D. J.
name: Johnson, M. W. G.
name: Johnson, M. D.
name: Krause, E.
name: Kuehn, K.
name: Kuropatkin, N.
name: Lima, M.
name: Maia, M. A. G.
name: Marshall, J. L.
name: Menanteau, F.
name: Miquel, R.
name: Mohr, J. J.
name: Nichol, R. C.
name: Nord, B.
name: Ogando, R.
name: Plazas, A. A.
name: Reil, K.
name: Romer, A. K.
name: Sanchez, E.
name: Scarpine, V.
name: Sevilla-Noarbe, I.
name: Smith, R. C.
name: Suchyta, E.
name: Tarle, G.
name: Thomas, D.
name: Thomas, R. C.
name: Tucker, D. L.
name: Weller, J.
Publisher: name: arXiv
Year: 2016
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $5σ$ 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 $\gtrsim 3σ$ 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.
description: 7 Pages, 2 Figures, 1 Table. Accepted to ApJL, updated to better match published version