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

TL;DR

The study introduces a Bayesian overlap framework that integrates remnant black-hole properties (final spin and recoil) with EM emission models to assess GW–EM coincidences in AGNs. Applied to GW190521–ZTF19abanrhr, it finds the Blandford–Znajek jet scenario strongly disfavoured unless the jet opening angle is unrealistically large, while a diffusive flare remains only modestly disfavoured under conservative assumptions. The analysis demonstrates how EM-informed priors and remnant-parameter constraints refine significance estimates beyond sky-location matches, and shows via simulations that the method can robustly boost true coincidences and suppress false ones as a function of SNR and geometry. Overall, the approach provides a physically grounded tool to probe flare mechanisms, AGN disk geometry, and multi-messenger associations in dense environments around merging black holes.

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 $Ω$ we obtain respective combined odds $\log_{10} {\cal O}_{Ω,{\rm jet}}= -1.17$ and $\log_{10} {\cal O}_{Ω,{\rm diff}}= +0.39$ 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.

Figures (5)

• Figure 1: Illustration of the different AGN flare mechanisms and their relations to the remnant BH properties. Left: The remnant BH must be kicked out of the AGN disk on the side facing the observer. Center: For a jet-like flare, the observer line-of-sight must be within the jet cone, which is closely aligned with the remnant spin axis. Right: in the case of a diffusive isotropic flare, the observer must not be behind the torus surrounding the disk.
• Figure 2: Constraints on the remnant parameters of GW190521 imposed by each emission mechanism. Two- and one-dimensional posterior distributions for the parameters $\theta_{aN},\ \iota$ and $\theta_{KN}$. The corresponding priors are shown in grey. Vertical dashed lines delimiting the symmetric 90% credible intervals around the median, which are quoted at the top of the plot. The two-dimensional contours denoting the $\{0.5, 1.0, 1.5, 2.0\}\,\sigma$ credible regions. White areas denote the regions allowed the AGN hypothesis under jet and diffusive flare hypotheses, with forbidden regions represented in red. Note that show only half of the domain for $\theta_{aN}$ to ease the visualisation of the allowed region, and naturally the vertical line in the top-left panel delimits the left-credible interval.
• Figure 3: Evidence for a true association under flare mechanisms for varying remnant parameters thresholds Inverse of the overlap integrals, in log scale $-\log_{10}{\cal I}$, as a function of varying thresholds on $\theta_{aN}$, $\iota$ and $\theta_{KN}$ The top panel corresponds to the BZ jet (Eq. \ref{['eq:caseI']}) while the lower panel corresponds to the diffusive flare.
• Figure 4: Identifying true Blandford-Znajek driven jets at high SNR. Different panels correspond to BBHs with different spin configurations. Blue (green) horizontal lines represent the symmetric $90\%$ credible intervals of $\overline\theta_{aN}$ obtained for simulated signals from BBHs with mass ratios $Q=1.5\ (3)$ injected in Advanced LIGO-Virgo. The median values are marked by the squares. For each injection, the overlap integral in support of the jet hypothesis is denoted by a star and plotted against the true value of $\theta_{aN}$. The white region $\theta_{aN} < 10^{\circ}$ corresponds to that allowed by the jet hypothesis.
• Figure 5: Overlap integrals results for SNR 15 and 30. Same as Fig. \ref{['fig:injection_overlap_extra']} but for cases with SNRs of 15 (left) and 30 (right).