General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes (1201.4163v3)

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]

• The paper demonstrates that magnetic flux saturation near rotating black holes produces non-axisymmetric, magnetically choked accretion flows that differ from standard MRI-driven models.
• The paper shows that under high spin conditions (|a/M| ≳ 0.9), jet efficiencies can exceed 100%, linking enhanced magnetization to powerful energy outflows.
• The paper introduces jet-disk quasi-periodic oscillations with quality factors around 100, providing a potential diagnostic tool for measuring black hole spins.

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

This paper presents comprehensive simulations within the framework of general relativistic magnetohydrodynamics (GRMHD) to evaluate properties and dynamics of magnetically choked accretion flows (MCAFs) around black holes. This paper addresses significant concepts involving the accumulation and effects of magnetic fields in accretion disks swirling around black holes (BHs). Unlike the conventional magnetorotational instability (MRI)-driven accretion flows that rely on weak magnetic fields, the MCAF state arises when substantial poloidal magnetic flux is drawn towards the BH, forming intense, non-axisymmetric structures that incite magnetic Rayleigh-Taylor instabilities.

Key Results and Analysis

1. Magnetic Flux Saturation: The paper demonstrates the mechanism through which magnetic flux accumulates and saturates near a rotating BH, independent of initial poloidal magnetic flux amounts. Notably, the flux reaches an equilibrium state governed by various instabilities such as magnetic Rayleigh-Taylor, generating non-axisymmetric accretion flows as opposed to classically symmetric MRI-driven flows.
2. Efficiencies and Accretion Dynamics: One of the significant outcomes revealed by the simulations is the identification of scenarios where BHs can attain jet efficiencies exceeding 100% for spins $|a/M| \gtrsim 0.9$. This efficiency emerges from enhanced magnetization over substantial portions of the event horizon, accompanying an intricate interplay between magnetic forces and ram pressures. Additionally, large scale magnetohydrodynamic interactions suppress the MRI, converting the accretion flow into a magnetically influenced state, radically different from MRI-dominated models.
3. Jet-Disk Quasi-Periodic Oscillations (JD-QPOs): The paper introduces a compelling mechanism for QPOs driven by the interaction between the jet and disk at the boundaries of the magnetosphere. These oscillations arise from the BH's inherent rotation frequency and are characterized by dominant spherical harmonic modes typically found in jet-disk interfaces. With measurable quality factors $Q \sim 100$ in jets, this discovery suggests intriguing possibilities for directly connecting observations of QPOs with BH spin measurements, providing a valuable diagnostic tool.
4. Toroidal Magnetic Field Dynamics: Simulations involving initially toroidal magnetic fields reveal the occasional spontaneous emergence of dipolar large-scale magnetic field configurations near the BH horizon, although these configurations only produce ephemeral and weakly powered jets due to low mass-loading. It proposes possibilities for jet formation through gradually acquired large-scale dipolar fields even in initially weak configurations.
5. Resolution and Convergence Analysis: The paper incorporates extensive convergence testing and resolution analysis under varying grid configurations, affirming robustness and reliability across various dimensions. The findings imply well-resolved, numerically stable data on masses, magnetic energies, and turbulent properties that contribute to the observed phenomena.

Implications and Future Directions

This investigation sheds light on the dynamic and structural complexities underpinning accretion flows impacted by magnetic fields, particularly in thick accretion disks around rapidly spinning BHs. The work not only expands our theoretical understanding of astrophysical phenomena such as jets and QPOs but posits frameworks that might reconcile discrepancies observed across galactic nuclei and BH x-ray binaries.

Moreover, detailed modeling of MCAF states could revolutionize our approach to modeling the innermost regions of accretion disks and their episodic variability seen in both environmental AGN jets and BH X-ray binaries. Future research might explore incorporating radiative transfers to evaluate observability conditions fully and assess different field geometries and disk configurations that could challenge or substantiate these insights. Additionally, enlightening the role of MCAFs in varied astrophysical contexts holds promise for holistic comprehension of BH-accretion systems and related relativistic jet phenomena.