Constraining binary mergers in AGN disks using the non-observation of lensed gravitational waves

TL;DR

This paper proposes using the absence of observed lensed gravitational waves to constrain the fraction of binary black holes in AGN disks, providing insights into their role in black hole mergers and the structure of accretion disks.

Contribution

It introduces a novel method to constrain the presence of BBHs in AGN disks through non-observation of lensing effects in gravitational waves.

Findings

Current data can start constraining BBH populations in AGN disks.

Future detectors will significantly limit the role of migration traps in BBH formation.

Non-detection of lensing can inform about the size of accretion disks.

Abstract

The dense and dynamic environments within active galactic nuclei (AGN) accretion disks may serve as prolific birthplaces for binary black holes (BBHs) and one possible origin for some of the BBHs detected by gravitational-wave (GW) observatories. We show that a considerable fraction of the BBH in AGN disks will be strongly lensed by the central supermassive black hole (SMBH). Thus, the non-observation of lensed GW signals can be used to constrain the fraction of BBH binaries residing in AGN disks. The non-detection of lensing with current ${\cal O}(100)$ detections will be sufficient to start placing constraints on the fraction of BBHs living within accretion disks near the SMBH. In the next-generation detectors era, with ${\cal O}(10^5)$ BBH observations and no lensed events, we will be able to rule out most migration traps as dominant birthplaces of BBH mergers; moreover, we will be able to constrain the minimum size of the accretion disk. On the other hand, should AGNs constitute a major formation channel, lensed events from AGNs will become prominent in the future.