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

          @misc{ pirsa_PIRSA:25030132,
            doi = {10.48660/25030132},
            url = {https://pirsa.org/25030132},
            author = {},
            keywords = {Strong Gravity},
            language = {en},
            title = {Hot, Retrograde Tilted MADs: Misaligned, Precessing, and Shaped by Electromagnetic Torques},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {mar},
            note = {PIRSA:25030132 see, \url{https://pirsa.org}}
          }
          

Abstract

Tilted magnetically arrested disks (MADs) around black holes—where strong magnetic fields regulate accretion and jets—exhibit striking alignment behavior dictated by black hole spin direction. Using 3D general-relativistic magnetohydrodynamic (GRMHD) simulations of tilted MADs, we find that prograde disks align via a two-stage process: an initial rapid alignment phase, ending at the flux saturation timescale, followed by a slower, spin-independent phase. In contrast, retrograde MADs remain persistently misaligned, with their inner disks precessing four times faster than weakly magnetized systems—a potential explanation for high-frequency quasi-periodic oscillations (QPOs). By analyzing magnetic and hydrodynamic torques within ideal GRMHD, we show that alignment in prograde disks is dominated by electromagnetic stresses from the magnetosphere. However, the same magnetic forces— which always act to align the disk with the black hole spin—are significantly weaker in retrograde disks, allowing opposing hydrodynamic torques to dominate. These results suggest that jets alone may not be sufficient to align MAD disks, instead highlighting the magnetosphere’s crucial role in mediating spin-disk coupling.