Tracing the light: Identification for the optical counterpart candidates of binary black-holes during O3

TL;DR

This study updates the search for electromagnetic counterparts to binary black hole mergers during O3, identifying two strong optical-GW associations in AGN, and uses these to estimate the Hubble constant with results consistent with previous measurements.

Contribution

It provides an updated analysis with additional data, identifying two robust optical-GW associations, and demonstrates the potential of multi-messenger observations for cosmological measurements.

Findings

Two AGN flares strongly correlated with GW events.

Hubble constant estimated as 72.1 km/s/Mpc with large uncertainties.

Refined H0 measurement to 73.5 km/s/Mpc when including GW170817.

Abstract

The accretion disks of active galactic nuclei (AGN) are widely considered the ideal environments for binary black hole (BBH) mergers and the only plausible sites for their electromagnetic (EM) counterparts. Graham et al.(2023) identified seven AGN flares that are potentially associated with gravitational-wave (GW) events detected by the LIGO-Virgo-KAGRA (LVK) Collaboration during the third observing run. In this article, utilizing an additional three years of Zwicky Transient Facility (ZTF) public data after their discovery, we conduct an updated analysis and find that only three flares can be identified. By implementing a joint analysis of optical and GW data through a Bayesian framework, we find two flares exhibit a strong correlation with GW events, with no secondary flares observed in their host AGN up to 2024 October 31. Combining these two most robust associations, we derive a Hubble constant measurement of $H_{0}= 72.1^{+23.9}_{-23.1} \ \mathrm{km \ s^{-1} Mpc^{-1}}$ and incorporating the multi-messenger event GW170817 improves the precision to $H_{0}=73.5^{+9.8}_{-6.9} \ \mathrm{km \ s^{-1} Mpc^{-1}}$. Both results are consistent with existing measurements reported in the literature.