Probing the Formation Environment of Strongly Lensed Black Hole Mergers: Implications for the AGN-disk Channel

TL;DR

This paper explores how gravitational lensing effects on black hole merger signals can reveal details about their environments, especially in AGN disks, helping to understand their formation mechanisms.

Contribution

It introduces a novel method to use lensing-induced Doppler shifts and accelerations to constrain the environment and dynamics of lensed GW sources, focusing on AGN disks.

Findings

AGN-disk black hole mergers show detectable lensing effects

Joint detection of Doppler shifts constrains source environment

Method applicable to upcoming GW lensing catalogs

Abstract

The observation of multiple images from a strongly lensed gravitational wave (GW) source provides the observer with a stereoscopic view of the source. This allows for a measure of its relative proper motion by comparing the induced GW Doppler shifts between the different images. In addition, if the GW source is in a dynamical environment it will be subject to an acceleration, which will show up as a time dependent Doppler shift in each individual image. In this work we quantify for the first time how a joint detection of these effects can be used to constrain the underlying dynamics and environment of the lensed GW source. We consider a range of different astrophysical environments, from massive clusters to stellar triples, and find that binary black hole (BBH) mergers in Active Galactic Nuclei disks (AGN-disks) are particularly likely to have orbital parameters that can be constrained through our considered lensing setup. Applying these methods to the upcoming catalog of cosmologically strongly lensed GW sources will open up new possibilities for probing their origin and underlying formation mechanisms.