General relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around black holes

TLDR

Ordered magnetic fields qualitatively affect black hole accretion flows and jets. The study investigates 3‑D global GRMHD simulations of geometrically thick, radially extended accretion flows around black holes with a range of spins and initial toroidal or poloidal magnetic field configurations. Simulations employ fully three‑dimensional general relativistic magnetohydrodynamics on disks with H/R ≈ 0.2–1 around black holes spanning a range of dimensionless spins, starting from toroidally or poloidally dominated magnetic fields. The results show that toroidal‑field models with high spin generate large‑scale dipolar flux that threads the black hole and launches transient jets, while poloidal‑field models form a magnetically choked accretion flow that suppresses MRI and, for high spin or low H/R, produces persistent Blandford–Znajek jets with.

Abstract

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|\sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($\phi$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough $|a/M|$, coherent large-scale (i.e. $\gg H$) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. For sufficiently high $|a/M|$ or low $|H/R|$ the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric "magnetically choked accretion flow" (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with $\gtrsim 100$% efficiency for $|a/M|\gtrsim 0.9$. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic $|m|=1$ mode period of $\tau\sim 70GM/c^3$ for $a/M\sim 0.9$ with coherence quality factors $Q\gtrsim 10$. [abridged]