Binary black holes in magnetized AGN disks

TL;DR

This work investigates how strong magnetic fields in AGN disks regulate accretion onto equal-mass binary black holes embedded in a local shearing-box. Using 3D ideal MHD simulations with vertical magnetic flux injection, it shows that dynamically important magnetic fields can drive well-collimated outflows (magnetic tower jets) that reach the disk’s vertical extent, with the outflow strength and presence strongly dependent on the binary’s radial position via the parameter $\lambda=R_H/a_b$. The results reveal an evolution from hydrodynamic-like accretion to magnetically dominated regimes, featuring toroidal-field amplification and episodic flux eruptions that modulate accretion and jet activity. These findings underscore the environment-dependent EM signatures of BBH mergers in AGN disks and provide a framework for linking local disk physics, magnetic flux dynamics, and potential multi-messenger observables in such systems.

Abstract

Stellar-mass binary black hole (BBH) mergers occurring within the disks of active galactic nuclei (AGN) are promising sources for gravitational waves detectable by the LIGO, Virgo, and KAGRA (LVK) interferometers. Some of these events have also been potentially associated with transient electromagnetic flares, indicating that BBH mergers in dense environments may be promising sources of multi-messenger signals. To investigate the prospects for electromagnetic emission from these systems, we study the dynamics of accretion flows onto BBHs embedded in AGN disks using numerical simulations. Although recent studies have explored this scenario, they often employ simplified disk models that neglect magnetic fields. In this work, we examine how strong magnetic fields influence and regulate the accretion onto such binary systems. In this context, we conduct three-dimensional magnetohydrodynamical local shearing-box simulations of a binary black hole system embedded within a magnetized disk of an AGN. We observe that the dynamically important magnetic fields can drive the formation of well-collimated outflows capable of penetrating the vertical extent of the AGN disk. However, outflow generation is not ubiquitous and strongly depends on the radial distance of the binary from the supermassive black hole (SMBH). In particular, binaries placed at a larger distance from the central SMBH show relatively more transient accretion and the formation of stronger spiral shocks. Furthermore, accretion behavior onto the binary system via individual circum-singular disks (CSDs) is also modulated by local AGN disk properties. Our simulations highlight the importance of shear velocity in the amplification of the toroidal magnetic field component, which plays a crucial role in governing the outflow strength.

Figures (11)

• Figure 1: Cartoon depiction of our model and simulation setup to study the evolution of BBH ($m_\text{1}$ and $m_\text{2}$) with a separation $a_\text{b}$, embedded in the disk of a SMBH of mass $M$. CoM of binary moves in a circular orbit in the $x-y$ plane.
• Figure 2: 2D slices for $\log_{10}\rho$ at $z=0$ plane are plotted at different times to show the overall morphology of accretion flow. The bottom panels show the zoomed region near the BBH to highlight the disk structure. The line contours for sonic Mach number $\mathcal{M}$ are also plotted in the bottom panels, white lines show $\mathcal{M}=1.$
• Figure 3: Density and plasma beta slices at $z=0$ plane in $\log_{10}$ scale are plotted at various epochs to show magnetic field accumulation in CSDs. The sink regions are masked by the black and green circles in the top and bottom rows for better illustration.
• Figure 4: Three-dimensional volume rendering of density overlaid with velocity streamlines, illustrating the spatial distribution of density structures of CSDs and the corresponding flow dynamics.
• Figure 5: 2D distribution of $\log_{10}\rho$ and $\log_{10}\beta$ along with velocity vectors are plotted in the $x-z$ plane ($y=0$) at different time stamps mentioned in the panels. The bottom row shows the ratio of toroidal and poloidal components of the magnetic field.