Magnetically Driven Retrograde Precession in Misaligned Black Hole Accretion Flows

TL;DR

This work investigates how misaligned black hole accretion flows precess under the combined influence of frame-dragging (Lense-Thirring) and large-scale magnetic fields. Using three-dimensional GRMHD simulations with tilted disks across HD, SANE, and MAD configurations, the authors show that strongly magnetized MAD flows can exhibit global retrograde precession driven by magnetic torques, dominating over LT precession when the magnetic flux is high. The precession direction, rate, and its radial dependence depend on disk size and magnetic state, with retrograde precession slowing and eventually ceasing as the disk expands. These findings provide a plausible magnetically driven mechanism for observed jet precession in systems like M87$^*$ and outline observable signatures to distinguish retrograde magnetic torques from LT-driven prograde precession using future interferometric arrays such as ngEHT and ngVLA.

Abstract

Observations of accreting black hole (BH) systems, such as microquasars and supermassive black holes, often reveal a precessing jet with changing directions, indicating a misaligned accretion flow relative to the BH spin. The precession is commonly attributed to the Lense-Thirring (LT) effect, which arises from the BH's rotation twisting the surrounding spacetime and accretion flow. In the strongly magnetized regime, which is preferred accretion flow conditions for M~87$^*$ and likely other jet-producing systems, the large-scale magnetic field can significantly influence the flow dynamics. Here, we perform large-scale three-dimensional general relativistic magnetohydrodynamic simulations of tilted accretion onto a rotating BH, and find a never-seen-before new retrograde precession. This precession arises from a magnetic torque on the disk generated by the poloidal magnetic field aligned with the BH's rotation, opposing the LT torque. This finding highlights the unique property of highly magnetized accretion flows around BHs and provides a new interpretation of jet precession observed in many systems.

Figures (9)

• Figure 1: Panels (a) and (b) are the time evolution of disk precession and tilt angles of the models T25a094HD (black), T25a094wSANE (yellow), T25a094sSANE (green), and T25a094nMAD (blue). Details on the notion of the models can be found in Table. \ref{['table:models']}.
• Figure 2: 3D Volume-rendering of the jet (colored by magnetization $\sigma$) and accretion disk (colored by density $\rho$) in the strongly-magnetized model T25a094nMADH, shown at two different time snapshots: (a) $t = 3{,}000\,\rm M$ and (b) $t = 17{,}160\,\rm M$. The retrograde precession of the jet-disk system is visible as a counter-rotation to the BH spin axis (positive $z-$direction).
• Figure 3: Evolution and radial profiles of jet and disk precession and tilt angles. Panels (a) and (b) depict the time evolution of the precession and tilt angles for the disk (black) and the jet (red) for the strongly-magnetized model T25a094nMAD. Panels (c) and (d) show the radial profiles of the disk’s precession and tilt angles for the model T25a094nMAD, with different colors indicating different averaging time ranges.
• Figure 4: Distributions of time-averaged logarithmic density on the $x-z$ plane of the rapidly-rotating BH model T25a094nMAD and non-rotating BH model T25a0nMAD with $a=0.9375$ and $a=0$ in panels (a) and (b), respectively. The streamlines in each panel are the time-averaged poloidal magnetic field lines. The time averaging range is from $t=14,000\,\rm M$ to $15,000\,\rm M$. The blue arrow in the zoomed-in part of panel (a) shows the BH spin direction.
• Figure 5: This figure presents the radial profiles of the ratio of the magnetic to LT precession rates, $|\Omega_{\rm mag}/\Omega_{\rm LT}|$, for simulations (a) $\tt T25a094nMAD$ (left) and (b) $\tt T25a094wSANE$ (right). Each colored line depicts the time-averaged profile over the interval indicated in the legend. These profiles are calculated from the simulation data by averaging values within radially binned shells, where each bin spans 8 cells in the radial direction.