The Evolution of Inclined Binary Black Holes in the Disks of Active Galactic Nuclei

Abstract

The accretion disks that fuel active galactic nuclei (AGNs) may house numerous stars and compact objects, formed in situ or captured from nearby star clusters. Embedded neutron stars and black holes may form binaries and eventually merge, emitting gravitational waves detectable by LIGO/VIRGO. AGN disks are a particularly promising environment for the production of high-mass gravitational-wave events involving black holes in the pair-instability mass gap, and may facilitate electromagnetic counterparts to black hole binary mergers. However, many orders of magnitude separate the typical length scales of binary formation and those on which gravitational waves can drive binary inspirals, making binary mergers inside the disk uncertain. Previous hydrodynamical simulations of binaries have either been restricted to two dimensions entirely, or focused on binaries aligned with the midplane of the disk. Herein we present the first three-dimensional, high-resolution, local-shearing-box, inviscid hydrodynamical simulations of disk-embedded binaries over a range of orbital inclinations. We find that retrograde binaries can shrink up to 4 times as quickly as prograde binaries, and that all binaries not perfectly aligned (or anti-aligned) with the AGN disk are driven into alignment. An important consequence of this is that initially retrograde binaries will traverse the inclinations where von Zeipel–Lidov–Kozai oscillations can drive binary eccentricities to large values, potentially facilitating mergers. We also find that interactions with the AGN disk may excite eccentricities in retrograde binaries and cause the orbits of embedded binaries to precess.

Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: Observational Characteristics in Different Regions of the Disks

“…Numerous compact objects in the AGN disk inevitably form binary systems (Tagawa et al 2020;Rowan et al 2023;Wang et al 2024) via Jacobi capture (Wang et al 2021b), GW emission (Rom et al 2024), dynamical friction (Qian et al 2024), or the collision of their circumsingle disks (J. . Simulations of BBH evolution show that the complex interactions between the BBH and AGN disk induce the contraction of the BBH separation Li & Lai 2022;Dittmann et al 2024), leading to BBH mergers (Bartos et al 2017;Yang et al 2019;Yi & Cheng 2019;McKernan et al 2020;Samsing et al 2022), often followed by electromagnetic flares (Wang et al 2021b;Chen & Dai 2024;Tagawa et al 2023c), such as GRBs ( Perna et al 2021a;Lazzati et al 2022;Yuan et al 2022;Ray et al 2023) and GRB afterglow (Wang et al 2022). Additionally, WD collisions (Luo et al 2023;Zhang et al 2023) and kilonova emission from NS-NS/BH-NS mergers (Zhu et al 2021b;Ren et al 2022;Kathirgamaraju et al 2023) are also important probes for studying compact objects in AGN disks.…”