Shock-induced partial alignment in geometrically-thick tilted accretion disks around black holes

TL;DR

This study uses 3D GRMHD simulations to explore how tilted accretion disks around spinning black holes develop partial alignment near the black hole due to standing shocks, with the process depending on black hole spin.

Contribution

It demonstrates that standing shocks in tilted thick disks induce partial inner disk alignment, and introduces a toy model predicting shock locations based on black hole spin and orbit crowding.

Findings

Shocks form near 6 gravitational radii in simulations.

Inner disk inclination decreases closer to the black hole.

Higher black hole spin increases the rate of partial alignment.

Abstract

We carry out idealized three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations of prograde, weakly magnetized, and geometrically thick accretion flows where the gas distribution is misaligned from the black hole spin axis. We evolve the disk for three black hole spins: $a = 0.5, 0.75$, and $0.9375$, and we contrast them with a standard aligned disk simulation with $a = 0.9375$. The tilted disks achieve a warped and twisted steady-state structure, with the outer disk misaligning further away from the black hole and surpassing the initial $24^\circ$ misalignment. However, closer to the black hole, there is evidence of partial alignment, as the inclination angle decreases with radius in this regime. Standing shocks also emerged in proximity to the black hole, roughly at $\sim$ 6 gravitational radii. We show that these shocks act to partially align the inner disk with the black hole spin. The rate of alignment increases with increasing black hole spin magnitude, but in all cases is insufficient to fully align the gas before it accretes. Additionally, we present a toy model of orbit crowding that can predict the location of the shocks in moderate-to-fast rotating black holes, illustrating a potential physical origin for the behavior seen in simulations\textemdash with possible applications in determining the positions of shocks in real misaligned astrophysical systems.