Electromagnetic vs. Lense-Thirring alignment of black hole accretion discs

TL;DR

This paper compares electromagnetic and Lense-Thirring torques in black hole accretion discs, showing how different disc states and magnetic field strengths influence alignment processes and implications for astrophysical observations.

Contribution

It demonstrates that BZ jet torque can dominate disc alignment over Lense-Thirring precession in various disc states, depending on density profiles and magnetic field strength.

Findings

Thin discs align at large radii before LT effects become significant.

Thick and super-Eddington discs align near the black hole in MAD states.

BZ jet torque influences black hole spin evolution and observational interpretations.

Abstract

Accretion discs and black holes (BHs) have angular momenta that are generally misaligned with respect to each other, which can lead to warps in the discs and bends in any jets produced. We consider a disc that is misaligned at large radii and torqued by Lense-Thirring (LT) precession and a Blandford-Znajek (BZ) jet torque. We consider a variety of disc states that include radiatively inefficient thick discs, radiatively efficient thin discs, and super-Eddington accretion discs. The magnetic field strength of the BZ jet is chosen as either from standard equipartition arguments or from magnetically arrested disc (MAD) simulations. We show that standard thin accretion discs can reach spin-disc alignment out to large radii long before LT would play a role, as caused by the slow infall time that gives even a weak BZ jet time to align the disc. We show that geometrically thick radiatively inefficient discs and super-Eddington discs in the MAD state reach disc-spin alignment near the black hole when density profiles are shallow as in magnetohydrodynamical simulations, while the BZ jet aligns discs with steep density profiles (as in advection-dominated accretion flows) with the BH spin out to larger radii. Our results imply that the BZ jet torque should affect the cosmological evolution of BH spin magnitude and direction, BH spin measurements in active galactic nuclei and X-ray binaries, and the interpretations for Event Horizon Telescope observations of discs or jets in strong-field gravity regimes.