Searching for electromagnetic emission in an AGN from the gravitational wave binary black hole merger candidate S230922g

TL;DR

This study conducted a long-term, multiwavelength search for electromagnetic signals associated with a gravitational wave binary black hole merger candidate within an active galactic nucleus, leading to the identification of promising counterpart candidates.

Contribution

First comprehensive multiwavelength monitoring of a GW BBH merger candidate in an AGN disk, revealing potential electromagnetic counterparts and unique spectral features.

Findings

Discovered 6 candidate counterparts, including AT 2023aagj.

Detected spectral asymmetries suggestive of off-center merger events.

First live search of its kind in an AGN environment.

Abstract

We carried out long-term monitoring of the LIGO/Virgo/KAGRA binary black hole (BBH) merger candidate S230922g in search of electromagnetic emission from the interaction of the merger remnant with an embedding active galactic nuclei (AGN) accretion disk. Using a dataset primarily composed of wide-field imaging from the Dark Energy Camera (DECam) and supplemented by additional photometric and spectroscopic resources, we searched ~ 70% of the sky area probability for transient phenomena, and discovered 6 counterpart candidates. One especially promising candidate - AT 2023aagj - exhibited temporally varying asymmetric components in spectral broad line regions, a feature potentially indicative of an off-center event such as a BBH merger. This represents the first live search and multiwavelength, photometric, and spectroscopic monitoring of a GW BBH optical counterpart candidate in the disk of an AGN.