Hydromagnetic flows from accretion discs and the production of radio jets

Abstract

We examine the possibility that energy and angular momentum are removed magnetically from accretion discs, by field lines that leave the disc surface and extend to large distances. We illustrate this mechanism by solving the equations of magnetohydrodynamics, assuming infinite conductivity, for axially symmetric, self-similar, cold magnetospheric flow from a Keplerian accretion disc in which the field strength B scales with radius r as Bo: ,-514 • We show that a centrifugally driven outflow of matter from the disc is possible, if the poloidal component of the magnetic field makes an angle of less than 60° with the disc surface. At large distances from the disc, the toroidal component of the magnetic field becomes important and collimates the outflow into a pair of anti-parallel jets moving perpendicular to the disc. Close to the disc, the flow is probably driven by gas pressure in a hot magnetically dominated corona. In this way, magnetic stresses can extract the angular momentum from a thin accretion disc and thus enable matter to be accreted, independently of the presence of viscosity. These jet solutions have the property that most of the power is concentrated within a central core, while most of the angular momentum and magnetic flux is carried near the jet walls. The relevance of this mechanism for the evolution of accretion discs around massive black holes in galactic nuclei and the production of jets in extragalactic radio sources is described.

OJ 287 as a Rotating Helix

“…Theoretical studies [11,12] and numerical simulations [13,14] show that the relativistic jets in AGN are likely to be driven by the rotating magnetic field trapped in the disk of a giant black hole. This can produce a jet of relativistically moving rotating plasma, threaded by a helical magnetic field.…”

Basic properties of Fermi blazars and the ‘blazar sequence’

“…Many models have been proposed to explain the origin of jets. In current theoretical models of the formation of jet, power is generated via accreting material and extraction of rotational energy of disc/black hole (Blandford & Znajek 1977;Blandford & Payne 1982), and then converted into the kinetic power of the jet. Our results show that the correlations between black hole mass and jet power for both Fermi and non-Fermi blazars are significant.…”

MHD Jets Around Galactic Objects

“…The jet velocity is always found to be a few times the escaping velocity of the central object (e.g., Livio, 1997). The most successful model, accounting for all known properties of observed jets, relies on a large scale magnetic field anchored on an accretion disc (Blandford & Payne, 1982). The field extracts the disc angular momentum, thereby allowing accretion, and accelerates a fraction of disc material to high speeds.…”