General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes

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]

💡 Research Summary

This paper presents a comprehensive suite of three‑dimensional global general‑relativistic magnetohydrodynamic (GRMHD) simulations aimed at understanding how ordered magnetic fields influence accretion flows and jet production around rotating black holes (BHs). The authors explore a wide parameter space: BH dimensionless spin |a/M| ranging from –0.99 to +0.99, disk thickness ratios H/R≈0.2–1, and two distinct initial magnetic topologies—toroidally dominated (azimuthal) and poloidally dominated (radial‑vertical).

In toroidal‑field models, when the spin magnitude exceeds a threshold (|a/M|≳0.7), large‑scale dipolar poloidal flux patches spontaneously emerge from the turbulent disk. These patches thread the event horizon, ignite transient relativistic jets via the Bland‑Znajek (BZ) mechanism, and then dissipate, causing the jets to switch off. Thus, toroidal configurations produce intermittent jet activity tied to the stochastic formation and destruction of flux patches.

Conversely, in poloidal‑field models the magnetic flux is efficiently advected inward and saturates near the BH. For sufficiently high spin (|a/M|≳0.9) or thin disks (H/R≲0.3), the accumulated polar magnetic pressure compresses the inflow into a geometrically thin, highly non‑axisymmetric structure the authors term a “magnetically choked accretion flow” (MCAF). Within the MCAF, the standard linear magnetorotational instability (MRI) is quenched, while magnetic Rayleigh‑Taylor and Kelvin‑Helmholtz instabilities develop at the interface between the compressed inflow and the expanding jet magnetosphere. This interface drives a novel jet‑disk quasi‑periodic oscillation (JD‑QPO) mechanism. The dominant mode is a spherical harmonic |m|=1 oscillation with a period τ≈70 GM/c³ for a/M≈0.9 and a quality factor Q≳10, matching observed high‑frequency QPOs in X‑ray binaries.

The MCAF state also creates a highly magnetized (β≲1) region covering most of the BH horizon, which maximizes the efficiency of the BZ process. The simulations report jet powers exceeding the rest‑mass energy inflow rate (efficiency η≳1, i.e., >100 %) for |a/M|≳0.9, indicating that the magnetic field can extract rotational energy from the BH more effectively than the accretion flow supplies it. Jets are strongly collimated, relativistic (Lorentz factors approaching unity in units of c), and persist as long as the magnetosphere remains saturated.

Overall, the study demonstrates that the initial magnetic topology, BH spin, and disk thickness jointly determine whether a system evolves into a transient, flux‑patch‑driven jet regime or a sustained, highly efficient BZ jet powered by a magnetically choked accretion flow. The identified JD‑QPO mechanism provides a plausible physical origin for the high‑frequency QPOs observed in many accreting BH systems, linking jet dynamics directly to disk‑magnetosphere interactions. The authors suggest that future work incorporating radiative transfer and electron thermodynamics will be essential for quantitative comparison with multi‑wavelength observations.