Kick & spin: new probes for multi-messenger black-hole mergers in AGNs

TL;DR

This paper introduces a statistical framework to evaluate the physical plausibility of electromagnetic counterparts to black-hole merger gravitational waves in AGNs, considering emission mechanisms and black-hole recoil and spin.

Contribution

It presents a novel evidence-based method that incorporates emission physics and black-hole parameters to assess GW-EM associations in multi-messenger astronomy.

Findings

The Blandford-Znajek jet model is strongly disfavoured for the GW190521-ZTF19abanrhr event.

The isotropic flare model is moderately disfavoured based on the evidence.

The combined odds suggest a possible true GW-EM coincidence, constrained by emission physics.

Abstract

Recoiling remnants of black-hole mergers in dense environments can produce bright electromagnetic (EM) counterparts to the gravitational-wave (GW) emission. Significance assessments of such GW-EM candidates are restricted to time and sky-localisation consistency, omitting the physics governing the EM emission process. Different emission mechanisms, however, impose different observability constraints on the remnant black-hole recoil and spin, which are gravitational-wave observables. We present a statistical framework that includes such parameters. We assess the consistency of the GW190521-ZTF19abanrhr pair with two types of emission processes: a Blandford-Znajek jet closely aligned with the final spin axis and a diffusive isotropic flare. Assuming the sky-location of ZTF19abanrhr, we find these mechanisms to be respectively strongly and moderately disfavoured with log-evidences $\log_{10}{\cal I}_{\rm jet} = -1.65$ and $\log_{10}{\cal I}_{\rm diff} = -0.075$. Combining these with odds for a common sky-location $\Omega$ we obtain respective combined odds $\log_{10} {\cal O}_{\Omega,{\rm jet}}= -1.17$ and $\log_{10} {\cal O}_{\Omega,{\rm diff}}= +0.39$ for a true GW-EM coincidence as opposed to a random one. Our method leverages a previously unexplored evidence axis to assess GW-EM associations and constrain both the physics powering flare mechanisms and the properties of AGNs.