Constraining the environment of compact binary mergers with self-lensing signatures

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Main Authors: Ubach, Helena, Gieles, Mark, Miralda-Escudé, Jordi
Format: Preprint
Published: 2025
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author Ubach, Helena
Gieles, Mark
Miralda-Escudé, Jordi
author_facet Ubach, Helena
Gieles, Mark
Miralda-Escudé, Jordi
contents Gravitational waves (GWs) from coalescing binary black holes (BBHs) can come from different environments. GWs interact gravitationally with astrophysical objects, which makes it possible to use gravitational lensing by objects in the same gravitational system (self-lensing) to learn about their environments. We quantify the probability of self-lensing through the optical depth $τ$ for the main channels of detectable GWs at frequencies $f_{\rm GW}\sim (1-10^3)\,{\rm Hz}$. We then analyze the detectability of the lensing effect (imprint). In star clusters, the probability of self-lensing by stellar-mass black holes (BHs) is low, $τ\simeq10^{-7}$, even when taking into account nearby BHs in resonant interactions, $τ\simeq 10^{-5}$. Additionally, the lensing imprint of a stellar-mass lens (diffraction and interference) is too marginal to be detectable by the LIGO-Virgo-KAGRA detectors and most Einstein Telescope signals. For a massive BH lens in the center of a cluster, the probability can reach $τ\simeq 10^{-4}$ either via von Zeipel-Lidov-Kozai induced mergers of BBHs orbiting a central massive BH, or BBHs formed as GW captures in single-single interactions in the Bahcall-Wolf cusp of a nuclear cluster. For self-lensing by a supermassive BH for BBHs in the migration trap of an active galactic nucleus (AGN) disk, $τ\simeq 10^{-2}$. The imprint of these massive lenses are multiple images that are already detectable. Moreover, self-lensed signals from AGN disks have a distinct linear polarization. The probability depends on the extent of the detectability through the threshold impact parameter $y_{\rm max}$, which can increase for future detectors. We conclude that constraining the environment of BBHs is possible by combining self-lensing imprints with other waveform signatures such as eccentricity and polarization.
format Preprint
id arxiv_https___arxiv_org_abs_2505_04794
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle Constraining the environment of compact binary mergers with self-lensing signatures
Ubach, Helena
Gieles, Mark
Miralda-Escudé, Jordi
High Energy Astrophysical Phenomena
Cosmology and Nongalactic Astrophysics
Gravitational waves (GWs) from coalescing binary black holes (BBHs) can come from different environments. GWs interact gravitationally with astrophysical objects, which makes it possible to use gravitational lensing by objects in the same gravitational system (self-lensing) to learn about their environments. We quantify the probability of self-lensing through the optical depth $τ$ for the main channels of detectable GWs at frequencies $f_{\rm GW}\sim (1-10^3)\,{\rm Hz}$. We then analyze the detectability of the lensing effect (imprint). In star clusters, the probability of self-lensing by stellar-mass black holes (BHs) is low, $τ\simeq10^{-7}$, even when taking into account nearby BHs in resonant interactions, $τ\simeq 10^{-5}$. Additionally, the lensing imprint of a stellar-mass lens (diffraction and interference) is too marginal to be detectable by the LIGO-Virgo-KAGRA detectors and most Einstein Telescope signals. For a massive BH lens in the center of a cluster, the probability can reach $τ\simeq 10^{-4}$ either via von Zeipel-Lidov-Kozai induced mergers of BBHs orbiting a central massive BH, or BBHs formed as GW captures in single-single interactions in the Bahcall-Wolf cusp of a nuclear cluster. For self-lensing by a supermassive BH for BBHs in the migration trap of an active galactic nucleus (AGN) disk, $τ\simeq 10^{-2}$. The imprint of these massive lenses are multiple images that are already detectable. Moreover, self-lensed signals from AGN disks have a distinct linear polarization. The probability depends on the extent of the detectability through the threshold impact parameter $y_{\rm max}$, which can increase for future detectors. We conclude that constraining the environment of BBHs is possible by combining self-lensing imprints with other waveform signatures such as eccentricity and polarization.
title Constraining the environment of compact binary mergers with self-lensing signatures
topic High Energy Astrophysical Phenomena
Cosmology and Nongalactic Astrophysics
url https://arxiv.org/abs/2505.04794