Hot, Retrograde Tilted Magnetically Arrested Disks: Misaligned, Precessing, and Shaped by Electromagnetic Torques | CU Experts

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 general relativistic magnetohydrodynamic (GRMHD) simulations to study the evolution of these accretion flows as a function of black hole spin and the 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 torques always act to align the disk with the black hole spin, but are countered by opposing hydrodynamic fluxes in retrograde flows. We further develop a preliminary empirical model to explain the cause of the two-stage prograde alignment and discuss the possibility of alignment in the retrograde MAD. The persistent misalignment and near rigid-body precession of retrograde MADs represent a qualitatively different regime from the alignment seen in prograde flows, with potential implications for the time-variable appearance of accreting black holes.

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