Hydromagnetic flows from accretion discs and the production of radio jets

TLDR

Accretion discs around massive black holes can launch jets, with hot magnetically dominated coronae likely driving the flow near the disc and the mechanism having implications for the evolution of discs and jet production in extragalactic radio sources. The study investigates whether magnetic field lines that extend from the disc surface can remove energy and angular momentum from accretion discs. The authors solve the ideal MHD equations for an axially symmetric, self‑similar, cold magnetospheric flow from a Keplerian disc, assuming infinite conductivity and a magnetic field scaling as B∝r^−5/4. The model shows that a centrifugally driven outflow arises when the poloidal field makes an angle <60° with the disc, the toroidal field collimates the flow into anti‑parallel jets, magnetic stresses extract angular momentum enabling accretion without viscosity, and the jets concentrate most power in a central core while angular momentum and flux are carried near the walls.

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 |$B\,\propto\,{r}^{-5/4}$. .| 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.