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

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 similar to 0.015-0.05) accretion discs around rapidly spinning (a similar to 0.9) black holes and tilted by 45 degrees-65 degrees. We show that the inner regions of the discs with h/r less than or similar to 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.