[Paper Review] GRMHD simulations of BH activation by small scale magnetic loops: Formation of striped jets and active coronae

Anna Chashkina, Omer Bromberg+1 more|arXiv (Cornell University)|Jun 29, 2021

Astrophysical Phenomena and ObservationsPhysics and Astronomy40 references55 citations

TL;DR

This study uses 2D and 3D GRMHD simulations to investigate black hole activation via accretion of small-scale magnetic loops with alternating polarity. It finds that such loops can launch quasi-striped jets with 10–20% of the power seen in single-loop models, provided they are larger than ~10rg and maintained within ~100rg of the black hole, while also driving cyclic accretion states and current sheet dissipation that may power hard X-ray coronae.

ABSTRACT

We have performed a series of numerical experiments aimed at studying the activation of Kerr black holes (BHs) by advection of small scale magnetic fields. Such configurations may potentially give rise to the formation of quasi-striped Blandford-Znajek jets. It can also lead to enhanced dissipation and generation of plasmoids in current sheets formed in the vicinity of the BH horizon, which may constitute a mechanism to power the hard X-ray emission seen in many accreting BH systems (a la lamppost models). Our analysis suggests that formation of quasi-striped jets with significant power may be possible provided loops with alternating polarity having sizes larger than $\sim 10 r_g$ or so can be maintained (either form sporadically or advected from outside) at a radius $\lesssim 10^2 r_g$. This conclusion is consistent with recent results of general relativistic force-free simulations. We also find that the accretion dynamics exhibits cyclic behaviour in MAD states, alternating between high accretion phases and quenched accretion phases during which the magnetosphere becomes force-free out to radii $\gtrsim 10r_g$. We suggest that such a behaviour should lead to notable variations of the observed luminosity and image of the inner disc (BH shadow image). Finally, we find that the transition between accreted loops on the BH gives rise to the formation of current sheets and energetic plasmoids on the jet boundary during intermittent periods when the jet becomes inactive, in addition to an equatorial current sheet that forms during peaks in the jet activity.

Motivation & Objective

• To investigate whether small-scale magnetic loops with alternating polarity can activate Kerr black holes and launch relativistic jets.
• To examine the formation of current sheets and plasmoids near the black hole horizon due to loop accretion.
• To assess the implications of cyclic accretion dynamics for observable features like luminosity variability and black hole shadow morphology.
• To compare GRMHD results with previous GRFFE simulations and evaluate the robustness of jet and dissipation formation under realistic conditions.
• To explore the viability of such configurations as a power source for compact coronae in lamppost models of hard X-ray emission.

Proposed method

• Performed 2D and 3D general relativistic magnetohydrodynamics (GRMHD) simulations using the HARMPI code in Boyer-Lindquist coordinates.
• Used initial conditions with a Fishbone & Moncrief torus and embedded magnetic loops (dipole, quadrupole, multiple loops) with alternating polarity.
• Simulated accretion of loops into the black hole’s ergosphere, tracking jet formation, magnetic dissipation, and current sheet development.
• Analyzed time-resolved data to identify cyclic accretion states and measured characteristic length scales of poloidal magnetic fields.
• Inferred plasmoid formation in 3D simulations via curvature radii of magnetic field lines within the ISCO.
• Compared results with prior GRFFE simulations to assess consistency and limitations of the GRMHD approach.

Experimental results

Research questions

• RQ1 Can accretion of small-scale magnetic loops with alternating polarity lead to the formation of quasi-striped jets with significant power?
• RQ2 What conditions are required for such loops to maintain jet power and avoid rapid dissipation?
• RQ3 How do cyclic accretion dynamics—alternating between high and quenched accretion phases—affect observable properties like luminosity and shadow image?
• RQ4 Where and how do current sheets and plasmoids form in the magnetosphere during loop accretion?
• RQ5 Can this mechanism naturally explain the compact, hard X-ray emitting coronae observed in many black hole systems?

Key findings

• Quasi-striped jets with 10–20% of the jet power from a single loop can form if magnetic loops with alternating polarity and size >~10rg are maintained within ~100rg of the black hole.
• Cyclic accretion dynamics emerge in MAD states, alternating between high accretion and quenched phases where the magnetosphere becomes force-free out to ≳10rg.
• During quenched phases, a thin equatorial current sheet forms, potentially inconsistent with EHT black hole shadow observations, and may enhance high-energy emission.
• Plasmoids form in current sheets at the jet boundary during loop reconnection and in the equatorial plane during episodic reconnection in MAD states, indicating a viable dissipation channel.
• In 3D simulations, plasmoid formation is inferred from magnetic field line curvature radii inside the ISCO, consistent with 2D observations.
• The results are qualitatively consistent with GRFFE simulations but are limited by resolution, lack of self-consistent cooling, and artificial plasma injection in GRMHD.