Tearing up the disc: how black holes accrete. (arXiv:1209.1393v1 [astro-ph.HE])

Tearing up the disc: how black holes accrete. (arXiv:1209.1393v1 [astro-ph.HE]):
We show that in realistic cases of accretion in active galactic nuclei or
stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central
regions of tilted accretion discs around spinning black holes into a set of
distinct planes with only tenuous flows connecting them. If the original
misalignment of the outer disc to the spin axis of the hole is $45^{\circ}
\lesssim \theta \lesssim 135^{\circ}$, as in $\sim 70$% of randomly oriented
accretion events, the continued precession of these discs sets up partially
counter-rotating gas flows. This drives rapid infall as angular momentum is
cancelled and gas attempts to circularize at smaller radii. Disc breaking close
to the black hole leads to direct dynamical accretion, while breaking further
out can drive gas down to scales where it can accrete rapidly. For smaller tilt
angles breaking can still occur, and may lead to other observable phenomena
such as QPOs. For such effects not to appear, the black hole spin must in
practice be negligibly small, or be almost precisely aligned with the disc.
Qualitatively similar results hold for any accretion disc subject to a forced
differential precession, such as an external disc around a misaligned black
hole binary.