McFACTS IV: Electromagnetic Counterparts to AGN Disk Embedded Binary Black Hole Mergers

The accretion disks of active galactic nuclei (AGN) are promising environments for producing binary black hole (BBH) mergers, which have been detected via gravitational waves (GW) with LIGO-Virgo-KAGRA (LVK). BBH mergers embedded in AGN disks are unique among GW formation channels in their generic ability to produce electromagnetic (EM) counterparts, via interactions between the merger remnant and the surrounding disk gas (though these are not always observable). While such mergers represent valuable multi-messenger sources, the lack of predictive statistical models in existing literature currently limits our ability to select possible EM counterparts with GW detections in archival data and in real time using time-domain surveys such as ZTF or LSST. Here, we employ the Monte Carlo For AGN Channel Testing and Simulation code (\texttt{McFACTS}\footnote{https://www.github.com/mcfacts/mcfacts}) to predict the bolometric luminosities of jets and shocks associated with LVK-detectable BBH merger remnants in AGN disks. \texttt{McFACTS} predicts the distribution of GW observables for an underlying BH population and disk model. In this work we present a new capability that simultaneously generates the distribution of bolometric EM luminosities corresponding to these predicted GW detections. We show that (i) migration traps in dense, Sirko-Goodman-like AGN disks efficiently drive hierarchical BH mergers, yielding high-mass, high-spin BH remnants capable of powering observable EM counterparts across merger generations; and ii) mergers embedded in sufficiently dense disks with chirp mass $\mathcal{M}\gtrsim40M_\odot$ are highly likely to yield observable EM counterparts for sufficiently long-lived disks and top-heavy BH initial mass functions.

💡 Research Summary

This paper presents the first population‑synthesis study of electromagnetic (EM) counterparts to binary black‑hole (BBH) mergers that occur inside active galactic nucleus (AGN) accretion disks. Using the Monte Carlo For AGN Channel Testing and Simulation framework (McFACTS), the authors extend the code (now version 0.4.0) to generate bolometric luminosities for two principal EM emission mechanisms: (1) shocks produced when the merger remnant receives a gravitational‑wave recoil kick and dissipates kinetic energy into the surrounding gas, and (2) relativistic jets launched by a rapidly spinning, accreting remnant black hole.

The baseline astrophysical model assumes a 10⁸ M⊙ supermassive black hole accreting at 10 % of the Eddington rate, surrounded by a Sirko‑Goodman thin disk with viscosity α = 0.01, aspect ratio h/r ≈ 0.03, radial extent 6–5 × 10⁴ r_g, and a lifetime of ≈0.7 Myr. Each simulated galaxy contains a nuclear star cluster (NSC) of mass 3 × 10⁷ M⊙, whose stellar‑mass black holes follow a top‑heavy power‑law mass function (index = 2, 10–40 M⊙). The code tracks the capture of these black holes by the disk, their migration, formation of migration traps, and hierarchical mergers within the traps.

Key results are:

1. Efficient hierarchical merging – In dense migration traps, black holes undergo repeated mergers, producing remnants with masses up to ~200 M⊙ and dimensionless spins χ ≈ 0.7–0.95. The high spin is a direct consequence of prograde, gas‑aligned accretion and successive mergers.

2. EM luminosity distribution – For remnants with chirp mass 𝓜 ≳ 40 M⊙, the simulated shock luminosities span 10⁴³–10⁴⁶ erg s⁻¹, while jet luminosities range from 10⁴³–10⁴⁵ erg s⁻¹, depending on recoil velocity, ambient density, and spin. The probability that a LVK‑detectable merger produces an observable EM counterpart exceeds 30 % when the disk is sufficiently dense and lives for ≳0.5 Myr, especially under a top‑heavy initial mass function.

3. Observational challenges – AGN intrinsic luminosities (10⁴²–10⁴⁷ erg s⁻¹) and variability can easily mask modest flares. Geometric considerations imply that only about one‑quarter of jet breakout events are visible to a distant observer (type‑I AGN viewed face‑on, breakout on the near side). Large GW localization volumes further complicate real‑time follow‑up.

The authors discuss how upcoming time‑domain facilities (LSST, ZTF, Vera C. Rubin Observatory) combined with improved GW detector networks could mitigate these issues by providing deeper, higher‑cadence coverage and tighter sky localizations. They also outline future work: incorporating full GRMHD jet physics, refining AGN variability models to reduce false‑positive rates, and developing machine‑learning pipelines for rapid EM counterpart identification.

Overall, the study delivers a statistically robust framework linking GW observables to expected EM signatures in the AGN channel, offering concrete guidance for multi‑messenger searches and highlighting the potential of such events to constrain cosmological parameters like the Hubble constant.