Misaligned Accretion and Jet Production

TL;DR

This paper proposes a mechanism where misaligned accretion discs around black holes produce dipole radiation from precessing annuli, which drives jets along the spin axis and explains observed phenomena like QPOs and high brightness temperatures.

Contribution

It introduces a novel dipole radiation mechanism from Lense-Thirring precessing disc annuli that can generate jets without extracting spin energy, differing from traditional models.

Findings

Dipole radiation peaks sharply at small radii, proportional to R^{-17/2}.

Mechanically drives jets along the black hole spin axis, similar to pulsar jets.

Provides a physical explanation for low-frequency QPOs in accretion systems.

Abstract

Disc accretion on to a black hole is often misaligned from its spin axis. If the disc maintains a significant magnetic field normal to its local plane, we show that dipole radiation from Lense-Thirring precessing disc annuli can extract a significant fraction of the accretion energy, sharply peaked towards small disc radii $R$ (as $R^{-17/2}$ for fields with constant equipartition ratio). This low-frequency emission is immediately absorbed by surrounding matter or refracted towards the regions of lowest density. The resultant mechanical pressure, dipole angular pattern, and much lower matter density towards 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 disc emission is modulated at Lense-Thirring frequencies near the inner edge, providing a physical mechanism for low-frequency QPOs. Dipole emission requires nonzero hole spin, but uses only disc accretion energy. No spin energy is extracted, unlike the Blandford-Znajek process. MHD/GRMHD formulations do not directly give radiation fields, but can be checked post-process for dipole emission and so self-consistency, given sufficient resolution. Jets driven by dipole radiation should be more common in AGN than in X-ray binaries, and in low accretion rate states than high, agreeing with observation. In non-black-hole accretion, misaligned disc annuli precess because of the accretor's mass quadrupole moment, similarly producing jets and QPOs.