Evidence for lensing of gravitational waves from LIGO-Virgo data

International audienceRecently, the LIGO-Virgo Collaboration (LVC) has concluded there is no evidence for lensed gravitational waves (GWs) in the first half of the O3 run [LIGO Scientific Collaboration and Virgo Collaboration, Search for lensing signatures in the gravitational-wave observations from the first half of LIGO-Virgo’s third observing run], claiming “We find the observation of lensed events to be unlikely, with the fractional rate at μ>2 being 3.3×10-4.” While we agree that the chance of an individual GW event being lensed at μ>2 is smaller than <10-3, the number of observed events depends on the product of this small probability times the rate of mergers at high redshift. Observational constraints from the stochastic GW background indicate that the rate of conventional mass binary black hole (BBH) mergers (8<M/M⊙<15) in the redshift range 1<z<2 could be as high as O(107) events per year, more than sufficient to compensate for the intrinsically low probability of lensing. To reach the LVC trigger threshold, these events require high magnification but would still produce up to 10–30 LVC observable events per year. Thus, all the LVC observed ordinary stellar mass BBH mergers from this epoch must be strongly lensed with as many as 106 yr-1 events lying below the current detection threshold. By adopting a low BBH coalescence rate at high redshift, LVC implicitly assume that lensed events are insignificant and, thus, incorrectly underestimate the distances of most BBH events and correspondingly overestimate masses by factors of 2–5. Furthermore, the LVC adopted priors on time delay for ideal circularly symmetric lenses are in tension with the known distribution of observed time delays of lensed quasars that require elliptical potentials with a broad spread of time delays. Pairs of events like GW190421_213856GW190910_112807 and GW190424_180648GW190910_112807, which are directly assigned a probability of zero by LVC, should instead be considered as candidate lensed BBH pairs, since their separation in time is typical of lensed quasars. Replacing the LVC model prior for the time delay distribution with the empirical quasar-based distribution reverses the LVC conclusions and says that a significant fraction of BBH pairs identified by LVC are viable multiply lensed events, including quadruple systems