Identifying Type II Strongly Lensed Gravitational-Wave Images in Third-Generation Gravitational-Wave Detectors

TL;DR

This paper proposes a method to identify type II strongly-lensed gravitational wave images by exploiting waveform mismatches, predicting high detection rates with third-generation detectors and enhancing lensing studies.

Contribution

It introduces a novel approach to distinguish type II lensed GW images using waveform mismatch analysis, improving detection prospects with future detectors.

Findings

Significant fraction of type II images can be distinguished at SNR ~30.

Predicted yearly detection rates are 172.2 (CE), 118.2 (ET), 27.4 (LIGO Voyager).

A notable percentage of detectable events are distinguishable via their type II images.

Abstract

Strong gravitational lensing is a gravitational wave (GW) propagation effect that influences the inferred GW source parameters and the cosmological environment. Identifying strongly-lensed GW images is challenging as waveform amplitude magnification is degenerate with a shift in the source intrinsic mass and redshift. However, even in the geometric-optics limit, type II strongly-lensed images cannot be fully matched by type I (or unlensed) waveform templates, especially with large binary mass ratios and orbital inclination angles. We propose to use this mismatch to distinguish individual type II images. Using planned noise spectra of Cosmic Explorer, Einstein Telescope and LIGO Voyager, we show that a significant fraction of type II images can be distinguished from unlensed sources, given sufficient SNR ($\sim 30$). Incorporating models on GW source population and lens population, we predict that the yearly detection rate of lensed GW sources with detectable type II images is 172.2, 118.2 and 27.4 for CE, ET and LIGO Voyager, respectively. Among these detectable events, 33.1%, 7.3% and 0.22% will be distinguishable via their type II images with a log Bayes factor larger than 10. We conclude that such distinguishable events are likely to appear in the third-generation detector catalog; our strategy will significantly supplement existing strong lensing search strategies.