Joint population and strong-lensing inference for resolved gravitational-wave events probes the black-hole merger rate beyond the peak of star formation

Abstract

Gravitational waves can be lensed by intervening potentials of various scales. Strong lensing leads to underestimated distances and overestimated masses, biasing astrophysical results if not accounted for. I present a novel analysis of the LIGO-Virgo-KAGRA catalog of binary black-hole mergers, simultaneously inferring (1) whether or not each event is strongly lensed, (2) their magnifications if so, and (3) the underlying merger population, using both parametric and nonparametric population models as well as two models for the lensing optical depth. Posterior lensing probabilities do not exceed 1% for any event, so population constraints are consistent with those assuming nondetection of strong lensing or that lensing never occurs. This includes multiple subpopulations over black-hole mass and a component with high aligned spins. Compared to standard analyses, however, there are reductions of order 10% in uncertainty on the redshift at which the merger rate peaks and an order of magnitude in high-redshift rate upper limits. Though modest, these are the first constraints using only resolved events at redshifts where current ground-based gravitational-wave detectors are usually insensitive, at and beyond the peak of star formation.