Transient gravitational-wave (GW) emission is expected from highly energetic astrophysical events such as stellar-core collapses and mergers of binary neutron stars (NSs). The Laser Interferometer Gravitational-wave Observatory (LIGO; Abbott et al. 2009; Harry et al. 2010) includes detectors located in the United States near Hanford, Washington (H1) and Livingston, LA (L1). A similarly designed Virgo (V1; Accadia et al. 2012; Virgo Collaboration 2009) detector is located in Italy near the city of Cascina. Each interferometer contains a pair of perpendicular arms, 4 km long in the LIGO detectors and 3 km in Virgo, whose effective optical path length is slightly altered by passing GW signals. Since 2007, LIGO and Virgo have coordinated operations and shared data, so the three sites operate as a single network of detectors seeking direct measurements of GW signals. A fourth site, GEO600 in Hannover, Germany (Grote et al. 2008), also shares data with LIGO and Virgo. During the 2009–2010 science run of the LIGO/Virgo network (Abadie et al. 2012e) we implemented low-latency searches for GW transients. The analysis software identified GW event candidates (“triggers”), estimated their statistical significance, and reconstructed likely source positions in approximately 10 minutes. Alert messages were transmitted to a network of electromagnetic observatories after the manual validation of the GW triggers with a total latency of∼ 30 minutes. The collection of optical telescopes, as well as the Swift satellite, LOFAR, and the Expanded Very Large Array (Lazio et al. 2012), provided target of opportunity follow-up observations to the GW triggers. In earlier publications, we described the search method and likely sources of both GW and emission measure (EM) transients (Abadie et al. 2012c, 2012d), as well as the results of the follow-up observations performed with the Swift satellite (Evans et al. 2012).

In this paper, we describe the data set collected with optical telescopes, detail the methods used to search the data for transients consistent with expected optical counterparts to GWs, and report the results of this analysis. In this first effort to use optical instruments to search for transients based on data from GW detectors, none of the GW triggers showed strong evidence for being astrophysical in nature. However, searching for transients in a large sky area is a challenging problem, and uncertainty in the expected light curve and spectrum of the sought optical counterpart makes the problem harder still. For