GW190521: a binary black hole merger inside an active galactic nucleus?

TL;DR

This paper reports the detection of GW190521, a massive binary black hole merger, and its probable electromagnetic counterpart in an active galactic nucleus, enabling a multi-messenger analysis and estimation of the Hubble constant.

Contribution

It presents the first association of GW190521 with an electromagnetic flare in an AGN, supporting the hypothesis that such mergers can occur inside active galactic nuclei.

Findings

Preferred association between GW190521 and the AGN flare with high statistical significance.

Estimated Hubble constant consistent with previous measurements.

Supports the possibility of black hole mergers occurring within active galactic nuclei.

Abstract

GW190521, the most massive binary black hole merger confidently detected by the LIGO-Virgo-KAGRA collaboration, is the first gravitational-wave observation of an intermediate-mass black hole. The signal was followed approximately 34 days later by flare ZTF19abanrhr, detected in AGN J124942.3+344929 by the Zwicky Transient Facility at the 78% spatial contour for GW190521s sky localization. Using the GWTC-2.1 data release, we find that the association between GW190521 and flare ZTF19abanrhr as its electromagnetic counterpart is preferred over a random coincidence of the two transients with a log Bayes factor of 8.6, corresponding to an odds ratio of $\sim$ 5400 to 1 for equal prior odds and $\sim$ 400 to 1 assuming an astrophysical prior odds of 1/13. Given the association, the multi-messenger signal allows for an estimation of the Hubble constant, finding $H_0 = 102^{+27}_{-25}\mathrm{\ km \ s^{-1} \ Mpc^{-1}}$ when solely analyzing GW190521 and $79.2^{+17.6}_{-9.6}\mathrm{\ km \ s^{-1} \ Mpc^{-1}}$ assuming prior information from the binary neutron star merger GW170817, both consistent with the existing literature.