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Electromagnetic vs. Lense-Thirring alignment of black hole accretion discs | | Peter Polko
; Jonathan C. McKinney
; | | Date: |
25 Dec 2015 | | 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. | | Source: | arXiv, 1512.7969 | | Services: | Forum | Review | PDF | Favorites | |
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