Disc Tearing and Bardeen-Petterson Alignment in GRMHD Simulations of
  Highly Tilted Thin Accretion Discs
  Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) tend to be
surrounded by geometrically thin, radiatively cooled accretion discs. According
to both theory and observations, these are -- in many cases -- highly
misaligned with the black hole spin axis. In this work we present the first
general relativistic magnetohydrodynamic simulations of very thin ($h/r \sim
0.015-0.05$) accretion discs around rapidly spinning ($a \sim 0.9$) black holes
and tilted by 45-65 degrees. We show that the inner regions of the discs with
$h/r \lesssim 0.03$ align with the black hole equator, though at smaller radii
than predicted by theoretical 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
at all tilt values the system produces a pair of relativistic jets. At small
distances the 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.
