Search for Lensed Gravitational Waves Including Morse Phase Information: An Intriguing Candidate in O2

TL;DR

This paper searches for strongly lensed gravitational wave signals in LIGO and Virgo data, utilizing Morse phase information to identify a candidate event that could be a multiple-image lensing system, offering potential insights into the source and lens properties.

Contribution

It introduces the use of Morse phase in gravitational wave lensing searches and reports a candidate event with complex lensing features, providing new methods and constraints for identifying lensed GW signals.

Findings

Identified a candidate lensed GW event with three images including known and subthreshold events.

Estimated a false alarm probability of ~10^-4 for the candidate based on parameter coincidence.

Suggested a complex lens system with at least five images, possibly involving a galaxy cluster lens.

Abstract

We search for strongly lensed and multiply imaged gravitational wave signals in the second observing run of Advanced LIGO and Advanced Virgo (O2). We exploit a new source of information, the so-called Morse phase, which further mitigates the search background and constrains viable lenses. The best candidate we find is consistent with a strongly lensed signal from a massive binary black hole (BBH) merger, with three detected images consisting of the previously catalogued events GW170104 and GW170814, and a subthreshold trigger, GWC170620. Given the number of BBH events detected so far, we estimate an overall false alarm probability $\sim 10^{-4}$ for the observed high degree of parameter coincidence between the three events. On the flip side, we measure the Morse phase differences which suggest a complex and atypical lens system, with at least five images including a magnified image at a local maximum of the Fermat potential. The low prior probability for multiple lensed images and the amount of fine tuning required in the lens model reduce the credibility of the lensing hypothesis. The long time delays between lensed images point toward a galaxy cluster lens with an internal velocity dispersion $\sigma \sim 650\,{\rm km/s}$, and the observed strain amplitudes imply a likely range $0.4 < z \lesssim 0.7$ for the source redshift. We provide an error ellipse of $\sim 16\,{\rm deg}^2$ for the sky location of the source together with additional specific constraints on the lens-host system, and encourage follow-up efforts to confirm or rule out any viable lens. If this is indeed a lensed event, successfully pinpointing the system would offer a unique opportunity to identify the host galaxy of a BBH merger, and even localize the source within it.