Numerical modelling of binary black hole dynamics in active galactic nuclei discs

Abstract

Binary black holes (BBH) embedded in active galactic nuclei (AGN) discs is a promising system for understanding the astrophysical phenomenon that can produce both gravitational wave (GW) and electromagnetic (EM) signals. This work addresses how the dense, dynamic conditions within such discs influence the evolution and eventual merger of binary black holes. The study is motivated by recent GW detections and the possibility of associatedEMcounterparts, which suggest that mergers withinAGNdiscs may be more common or more easily observable than previously thought. Previous research has often relied on simplified models that do not fully capture the complexity of binary–disc interactions in these environments. To address this, we employ high-resolution hydrodynamical simulations to investigate the impact of mass ratio, orbital orientation and accretion dynamics on BBH systems within AGN discs. The simulations incorporate a systematic post-processing framework to quantify key parameters including mass accretion, torque and minidisc mass, to comprehensive model the binary’s secular orbital evolution. Our results reveal that the orientation of the binary with respect to the disc and the mass ratio of the components significantly affect the transfer of angular momentum, accretion variability and minidisc structure around each black hole. The mean mass accretion rate onto the binary system exhibits dependence on the accretor’s mass. These inflows display substantial variability, with their primary modulation frequency corresponding to the binary’s orbital period for circular orbital configurations. We find that the smaller/secondary black hole often dominates accretion, especially in systems with unequal masses and that retrograde (oppositely aligned to the disc flow) binaries experience more chaotic flows and stronger angular momentum loss than prograde systems. Our results reveal that the behaviour of embedded within AGN discs deviates significantly from that of binaries evolving in isolated circumbinary environments. Moreover, we find that the orbital hardening of these binaries proceeds on time-scales considerably shorter than their migration through the disc. These findings have important implications for interpreting GWevents, predicting EM and understanding the role of gas-rich environments in driving binary black hole mergers. This work contributes to a more complete picture of mergers in AGN and informs future multi-messenger (MM) observational strategies.