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Misaligned Accretion and Jet Production

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journal contribution
posted on 03.05.2018, 10:32 by Andrew King, Chris Nixon
Disk accretion onto a black hole is often misaligned from its spin axis. If the disk maintains a significant magnetic field normal to its local plane, we show that dipole radiation from Lense–Thirring precessing disk annuli can extract a significant fraction of the accretion energy, sharply peaked toward small disk radii R (as R −17/2 for fields with constant equipartition ratio). This low-frequency emission is immediately absorbed by surrounding matter or refracted toward the regions of lowest density. The resultant mechanical pressure, dipole angular pattern, and much lower matter density toward the rotational poles create a strong tendency to drive jets along the black hole spin axis, similar to the spin-axis jets of radio pulsars, also strong dipole emitters. The coherent primary emission may explain the high brightness temperatures seen in jets. The intrinsic disk emission is modulated at Lense–Thirring frequencies near the inner edge, providing a physical mechanism for low-frequency quasi-periodic oscillations (QPOs). Dipole emission requires nonzero hole spin, but uses only disk accretion energy. No spin energy is extracted, unlike the Blandford–Znajek process. Magnetohydrodynamic/general-relativistic magnetohydrodynamic (MHD/GRMHD) formulations do not directly give radiation fields, but can be checked post-process for dipole emission and therefore self-consistency, given sufficient resolution. Jets driven by dipole radiation should be more common in active galactic nuclei (AGN) than in X-ray binaries, and in low accretion-rate states than high, agreeing with observation. In non-black hole accretion, misaligned disk annuli precess because of the accretor's mass quadrupole moment, similarly producing jets and QPOs.

History

Citation

Astrophysical Journal Letters, 2018, 857:L7

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Astrophysical Journal Letters

Publisher

American Astronomical Society

issn

2041-8205

eissn

2041-8213

Acceptance date

19/03/2018

Copyright date

2018

Available date

03/05/2018

Publisher version

http://iopscience.iop.org/article/10.3847/2041-8213/aab8f9/meta

Language

en