Hot, Retrograde Tilted MADs: Misaligned, Precessing, and Shaped by Electromagnetic Torques

TL;DR

This study uses 3D GRMHD simulations to explore how tilted, magnetically arrested disks around black holes evolve, revealing distinct behaviors for prograde and retrograde orientations and their potential link to X-ray binary oscillations.

Contribution

It provides new insights into the evolution and alignment processes of tilted MADs, especially highlighting the persistent misalignment and precession of retrograde disks.

Findings

Prograde MADs align with black hole spin via a two-stage process.

Retrograde MADs remain misaligned and precess at high rates.

Electromagnetic torques tend to align disks, but hydrodynamic fluxes oppose this in retrograde flows.

Abstract

Tilted accretion disks in the magnetically arrested (MAD) state may be present in X-ray binaries and active galactic nuclei such as Sgr A* and M87. We have carried out 3D global GRMHD simulations to study the evolution of these accretion flows as a function of black hole spin and misalignment angle. Prograde MADs align with the spin through a two-stage process: an initial rapid alignment phase that operates on the magnetic flux saturation timescale, followed by a slower, spin-independent phase. In contrast, retrograde MADs remain persistently misaligned regardless of the black hole spin, displaying solid-body precession at rates four times higher than weakly magnetized flows at the same spin magnitude. By deriving torque equations in ideal GRMHD and evaluating them in a frame aligned with instantaneous disk orientation, we demonstrate that electromagnetic (EM) torques always act to align the disk with the BH spin, but are countered by opposing hydrodynamic fluxes in retrograde flows. We further develop a preliminary empirical model to explain the cause of two-stage prograde alignment and discuss the possibility of alignment in the retrograde MAD. Strongly magnetized, retrograde, misaligned accretion disks provide a candidate scenario for the low-frequency quasi-periodic oscillations in black hole X-ray binaries.