GWTC-4.0: Searches for Gravitational-Wave Lensing Signatures

TL;DR

This paper reports on searches for gravitational-wave lensing in LIGO-Virgo-KAGRA data, finding no evidence for lensing but setting constraints on event rates and analyzing a notable outlier event.

Contribution

It introduces new methods for detecting gravitational-wave lensing signatures and applies them to recent observational data, providing the first constraints on lensing rates in this context.

Findings

No evidence for strongly lensed gravitational-wave signals

Constraints on the rate of detectable strongly lensed events

Analysis of an intriguing outlier event GW231123_135430

Abstract

Gravitational waves can be gravitationally lensed by massive objects along their path. Depending on the lens mass and the lens--source geometry, this can lead to the observation of a single distorted signal or multiple repeated events with the same frequency evolution. We present the results for gravitational-wave lensing searches on the data from the first part of the fourth LIGO--Virgo--KAGRA observing run (O4a). We search for strongly lensed events in the newly acquired data by (1) searching for an overall phase shift present in an image formed at a saddle point of the lens potential, (2) looking for pairs of detected candidates with consistent frequency evolution, and (3) identifying sub-threshold counterpart candidates to the detected signals. Beyond strong lensing, we also look for lensing-induced distortions in all detected signals using an isolated point-mass model. We do not find evidence for strongly lensed gravitational-wave signals and use this result to constrain the rate of detectable strongly lensed events and the merger rate density of binary black holes at high redshift. In the search for single distorted lensed signals, we find one outlier: GW231123_135430, for which we report more detailed investigations. While this event is interesting, the associated waveform uncertainties make its interpretation complicated, and future observations of the populations of binary black holes and of gravitational lenses will help determine the probability that this event could be lensed.