Improved statistic to identify strongly lensed gravitational wave events

Abstract

ABSTRACT As the number of detected gravitational wave sources increases with increased sensitivity of the gravitational wave observatories, observing strongly lensed pairs of events will become a real possibility. Lensed gravitational wave (GW) events will have very accurately measured time delays and magnification ratios. Suppose we identify the lens system corresponding to a GW event in the electromagnetic domain and also measure the redshifts of the lens and the host galaxy; in that case, we can use the GW event to constrain important astrophysical parameters of the lens system. As most lensing events have image separations that are significantly smaller than the GW event localization uncertainties, we must develop diagnostics that will aid in the robust identification of such lensed events. We define a new statistic based on the joint probability of lensing observables that can be used to discriminate lensed pairs of events from the unlensed ones. To this end, we carry out simulations of lensed GW events to infer the distribution of the relative time delays and relative magnifications subdivided by the type of lensed images. We compare this distribution to a similar one obtained for random unlensed event pairs. Our statistic can improve the search pipelines’ existing ranking approach to down-select event pairs for joint parameter estimates. The distributions we obtain can further be used to define more informative priors in joint parameter estimation analyses for candidate lensed events.