Determination of Lens Mass Density Profile from Strongly-Lensed Gravitational-Wave Signals

TL;DR

This paper introduces a rapid model selection methodology for identifying the mass density profile of gravitational lenses from strongly-lensed gravitational wave signals, validated with simulations and a real event analysis.

Contribution

It presents a new, efficient approach to distinguish between different lens models in gravitational wave lensing, integrated into the Gravelamps software package.

Findings

The methodology successfully recovers lens models from simulated signals.

Applied to GW event pair, it favors the singular isothermal sphere model.

Results show wider posteriors for real events, consistent with non-lensed signals.

Abstract

As the interferometers detecting gravitational waves are upgraded, improving their sensitivity, the probability of observing strong lensing increases. Once a detection is made, it will be critical to gain as much information as possible about the lensing object from these observations. In this work, we present a methodology to rapidly perform model selection between differing mass density profiles for strongly lensed gravitational wave signals, using the results of the fast strong lensing analysis pipeline GOLUM. We demonstrate the validity of this methodology using some illustrative examples adopting the idealised singular isothermal sphere and point mass lens models. We take several simulated lensed signals, analyse them with GOLUM and subject them to our methodology to recover both the model and its parameters. To demonstrate the methodology's stability, we show how the result varies with the number of samples used for a subset of these injections. In addition to the analysis of simulations, we also apply our methodology to the gravitational wave event pair GW191230--LGW200104, two events with similar frequency evolutions and sky locations, which was analysed in detail as a potential lensing candidate but ultimately discarded when considering the full population and the uncertain nature of the second event. We find a preference for the singular isothermal sphere model over the point mass, though our posteriors are much wider than for the lensed injections, in line with the expectations for a non-lensed event. The methodology developed in this work is made available as part of the Gravelamps package of software.