Detectability of microlensed gravitational waves

TL;DR

This paper investigates the effects of microlensing on strongly-lensed gravitational waves, focusing on type-II images, and assesses detection strategies using Bayesian parameter estimation with simplified lens models.

Contribution

It provides a detailed analysis of microlensing effects on type-II images and evaluates the effectiveness of isolated point mass templates for detection.

Findings

Type-II images are typically demagnified and show larger waveform mismatches.

Isolated point mass templates are effective for detecting type-II microlensed signals.

More realistic templates are needed for accurate detection of type-I microlensed signals.

Abstract

Gravitational lensing describes the bending of the trajectories of light and gravitational waves due to the gravitational potential of a massive object. Strong lensing by galaxies can create multiple images with different overall amplifications, arrival times, and image types. If, furthermore, the gravitational wave encounters a star along its trajectory, microlensing will take place. Previously, it has been shown that the effects of microlenses on strongly-lensed type-I images could be negligible in practice, at least in the low magnification regime. In this work, we study the same effect on type-II strongly-lensed images by computing the microlensing amplification factor. As opposed to being magnified, type-II images are typically demagnified. Moreover, microlensing on top of type-II images induces larger mismatches with un-microlensed waveforms than type-I images. These results are broadly consistent with recent literature and serve to confirm the findings. In addition, we investigate the possibility of detecting and analysing microlensed signals through Bayesian parameter estimation with an isolated point mass lens template, which has been adopted in recent parameter estimation literature. In particular, we simulate gravitational waves microlensed by a microlens embedded in a galaxy potential near moderately magnified type-I and II macroimages, with variable lens masses, source parameters and macromagnifcations. Generally, an isolated point mass model could be used as an effective template to detect a type-II microlensed image but not for type-I images, demonstrating the necessity for more realistic microlensing search templates.