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

TL;DR

This work addresses the problem of predicting electromagnetic (EM) counterparts to binary black hole mergers embedded in active galactic nuclei (AGN) disks, a promising multi-messenger channel for LVK detections. The authors introduce McFACTS v0.4.0, a fast population-synthesis code that jointly models gravitational-wave observables and bolometric EM luminosities from merger remnants interacting with AGN disk gas, including jet (BZ/MAD) and shock emission mechanisms. They demonstrate that migration traps and dense, long-lived disks can drive hierarchical mergers producing high-mass, high-spin remnants capable of powering observable EM counterparts, while shock signatures are typically subdominant in standard Sirko–Goodman disks. The results yield quantitative predictions for the fraction and timing of observable EM flares as a function of chirp mass and disk/IMF parameters, offering practical guidance for future follow-up campaigns with LSST and UVEX and informing constraints on AGN disk and NSC properties in the LVK era and beyond.

Abstract

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.

Figures (12)

• Figure 1: Jet Breakout Time for Representative Luminosities as a Function of Remnant Location. Regions of the disk where, for a given a luminosity, a jet is able to break out of the disk before the engine turns off ($t_{\rm bo} < T_{\rm jet}$). Breakout times are denoted by the color bar. Our fiducial jet lifetime $T_{\rm jet} = 10^{7}$ s is annotated as a dashed black line. For our fiducial jet lifetime, any jet with a luminosity and radius lying to the left of the dotted line will breakout. Any jet whose luminosity and launching radius place it to the right of the dashed line will be muffled, escaping the disk only on the photon-diffusion timescale.
• Figure 2: Jet Luminosity as a Function of Radius in the Disk and Its Distribution in a SG Disk.Left panel: Bolometric jet luminosity (erg s$^{-1}$) vs. radius in disk (R$_{\mathrm{g}}$) calculated for each merger in McFACTS. Gold represents 1g-1g mergers, purple represents 2g-mg (m $\leq$ 2), and red represents 3g-ng (n $\in \mathbb{Z}^+$), where 1g is first generation BH (never experienced a merger), 2g is second generation (previously experienced one merger), etc. Right panel: Number of BBH mergers per generation of BH as a function of jet luminosity. This distribution represents mergers in 100 galaxies over 0.7 Myr, with a Sirko-Goodman disk model around a M$_\mathrm{SMBH} = 10^{8} ~\mathrm{M}_{\odot}$. The corresponding merger rate is $\mathcal{R}_{\rm GW}\sim11$ Gpc$^{-3}$ yr$^{-1}$ The input file for this figure is paper$\_$em/pem$\_$sg$\_$default.ini with galaxy num = 100 and seed = 3456789018. (also see Paper I)
• Figure 3: Jet breakout time as a Function of Jet luminosity. Our fiducial jet lifetime, $T_{jet} = 10^7$ s, is indicated with a solid green line. If the breakout time is shorter than the jet lifetime, the jet is expected to break out of the disk and its emission can be potentially observable. If the breakout time is longer than the jet lifetime, then the jet emission escapes the disk on the photon diffusion time, and it is practically undetectable.
• Figure 4: Shock Luminosity as a Function of Radius in the Disk and its Distribution in the Fiducial Model.Left panel: Bolometric shock luminosity (erg s$^{-1}$) as a function of disk radius (R$_{\mathrm{g}}$) for each merger in McFACTS. Right panel: Number of BBH mergers per BH generation as a function of shock luminosity. This distribution corresponds to mergers occurring in 100 galaxies over 0.7 Myr, assuming a Sirko-Goodman disk model around a M$_\mathrm{SMBH} = 10^{8} ~\mathrm{M}_{\odot}$. The input file for this figure is paper$\_$em/pem$\_$sg$\_$default.ini.
• Figure 5: Effects of The Disk Lifetime. Bolometric jet luminosity (erg s$^{-1}$) as a function of merger time (Myr), for disks with lifetimes $\tau_{\mathrm{AGN}} = 0.25$, $0.7$, and $1$ Myr (left to right). The corresponding merger rates are $\mathcal{R}_{\rm GW} \sim 1 ~{\rm Gpc}^{-3} {\rm yr}^{-1}, \mathcal{R}_{\rm GW} \sim 11 ~{\rm Gpc}^{-3} {\rm yr}^{-1}, {\rm ~and ~} \mathcal{R}_{\rm GW} \sim 23 ~{\rm Gpc}^{-3} {\rm yr}^{-1}$.