The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei

Abstract

Stellar-mass black holes can become embedded within the gaseous disks of active galactic nuclei (AGNs). Afterwards, their interactions are mediated by their gaseous surroundings. In this work, we study the evolution of stellar-mass binary black holes (BBHs) embedded within AGN disks using a combination of three-dimensional hydrodynamic simulations and analytic methods, focusing on environments in which the AGN disk scale height H is the BBH sphere of influence. We model the local surroundings of the embedded BBHs using a wind tunnel formalism and characterize different accretion regimes based on the local properties of the disk, which range from wind-dominated to quasi-spherical. We use our simulations to develop prescriptions for mass accretion and drag for embedded BBHs. We use these prescriptions, along with AGN disk models that can represent the Toomre-unstable outer regions of AGN disks, to study the long-term evolution of the BBHs as they migrate through the disk. We find that BBHs typically merge within 5 − 30 Myr, increasing their mass significantly in the process, allowing BBHs to enter (or cross) the pair-instability supernova mass gap. The rate at which gas is supplied to these BBHs often exceeds the Eddington limit, sometimes by several orders of magnitude. We conclude that most embedded BBHs will merge before migrating significantly in the disk. Depending on the conditions of the ambient gas and the distance to the system, LISA can detect the transition between the gas-dominated and gravitational wave dominated regime for inspiraling BBHs that are formed sufficiently close to the AGN ( 0.1 pc). We also discuss possible electromagnetic signatures during and following the inspiral, finding that it is generally unlikely but not inconceivable for the bolometric luminosity of the BBH to exceed that of the host AGN.

Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion

“…When this happens, the embedded BHs can migrate, accrete at super-Eddington rates, and potentially produce electromagnetic transients. The accretion geometry of the sBH-AGN binary depends mainly on the mass ratio of the system and the AGN disk scale height (Kaaz et al 2021). If the mass ratio is large, the system resembles more traditional circumbinary disks, where the sBH can impact the structure of the host disk (Baruteau et al 2011;Li et al 2021).…”

“…A black hole (BH) traversing through a gaseous medium is common in a wide variety of astrophysical scenarios, including common envelope evolution (MacLeod & Ramirez-Ruiz 2015;MacLeod et al 2017;Murguia-Berthier et al 2017;Everson et al 2020), wind accretion in binary systems (El Mellah & Casse 2015;El Mellah et al 2018), and the passage of stellar-mass black holes through the disks of active galactic nuclei (Stone et al 2017;Kaaz et al 2021). As the BH travels through the medium, it will accrete mass and potentially produce energetic outflows that alter its environment.…”

High Energy Neutrinos from Choked Gamma-Ray Bursts in AGN Accretion Disks

“…Besides classical GRBs from core collapse of massive stars, neutron star mergers and binary BH mergers (e.g., Bartos et al 2017;Kaaz et al 2021;Kimura et al 2021), accretion of a single BH (Wang et al 2021) and accretion-induced collapse of NSs (Perna et al 2021b) embedded within AGN disks were also studied recently. Such jets driven by embedded AGN objects could potentially be choked as well and hence produce high-energy neutrinos.…”

Hot Circumsingle Disks Drive Binary Black Hole Mergers in Active Galactic Nucleus Disks

“…Most studies for binary mergers in AGN disks are based on N -body (e.g., Secunda et al 2019), Monte Carlo (e.g., Yang et al 2019;Tagawa et al 2020;McKernan et al 2021) or hydrodynamical simulations (Baruteau et al 2011;Kaaz et al 2021). All of these works assume that BBHs contract in AGN disks.…”