Disc tearing and Bardeen–Petterson alignment in GRMHD simulations of highly tilted thin accretion discs

• Published in: Monthly notices of the Royal Astronomical Society Vol. 507; no. 1; pp. 983 - 990
• Main Authors: Liska, M, Hesp, C, Tchekhovskoy, A, Ingram, A, van der Klis, M, Markoff, S B, Van Moer, M
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.10.2021
• Subjects: MHD; black hole physics; galaxies: jets; methods: numerical; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/staa099

ABSTRACT Luminous active galactic nuclei and X-ray binaries often contain geometrically thin, radiatively cooled accretion discs. According to theory, these are – in many cases – initially highly misaligned with the black hole equator. In this work, we present the first general relativistic magnetohydrodynamic simulations of very thin (h/r ∼ 0.015–0.05) accretion discs around rapidly spinning (a ∼ 0.9) black holes and tilted by 45°–65°. We show that the inner regions of the discs with h/r ≲ 0.03 align with the black hole equator, though out to smaller radii than predicted by analytic work. The inner aligned and outer misaligned disc regions are separated by a sharp break in tilt angle accompanied by a sharp drop in density. We find that frame dragging by the spinning black hole overpowers the disc viscosity, which is self-consistently produced by magnetized turbulence, tearing the disc apart and forming a rapidly precessing inner sub-disc surrounded by a slowly precessing outer sub-disc. We find that the system produces a pair of relativistic jets for all initial tilt values. At small distances, the black hole launched jets precess rapidly together with the inner sub-disc, whereas at large distances they partially align with the outer sub-disc and precess more slowly. If the tearing radius can be modeled accurately in future work, emission model independent measurements of black hole spin based on precession-driven quasi-periodic oscillations may become possible.