Identifying the host galaxy of gravitational wave signals

One of the goals of the current LIGO-GEO-Virgo science run is to identify transient gravitational wave (GW) signals in near real time to allow follow-up electromagnetic (EM) observations. An EM counterpart could increase the confidence of the GW detection and provide insight into the nature of the source. Current GW-EMcampaigns target potential host galaxies based on overlapwith theGW sky error box.We propose a new statistic to identify the most likely host galaxy, ranking galaxies based on their position, distance, and luminosity. We test our statistic with Monte Carlo simulations of GWs produced by coalescing binaries of neutron stars and black holes, one of the most promising sources for ground-based GW detectors. Considering signals accessible to current detectors, we find that when imaging a single galaxy, our statistic correctly identifies the true host 20% to 50% of the time, depending on the masses of the binary components.With five narrow-field images the probability of imaging the true host increases from 50% to 80%. When collectively imaging groups of galaxies using large field-of-view telescopes, the probability improves from 30% to 60% for a single image and from 70% to 90% for five images. For the advanced generation of detectors (circa 2015þ ), and considering binaries within 100Mpc (the reach of the galaxy catalogue used), the probability is 40% for one narrow-field image, 75% for five narrow-field images, 65% for one wide-field image, and 95% for five wide-field images, irrespective of binary type.