Magnetic Fields in the Relativistic Jets of Active Galactic Nuclei

Abstract

AbstractAn abundance of information about the magnetic (B) fields of relativistic AGN jets has been obtained through radio polarization observations made on a wide range of scales, from the parsec scales probed by Very Long Baseline Interferometry to the kiloparsec scales probed by instruments such as the the Jansky Very Large Array and e-MERLIN. The observed radio emission is synchrotron radiation, for which the linear polarization angles in optically thin regions is orthogonal to the local synchrotron B fields. The characteristic B field structures observed for these jets on parsec scales are described. A key question is whether B field structures observed in particular AGN jets come about primarily due to “global” effects such as the intrinsic B field of the jet itself, which is expected to be helical, or local phenomena such as shocks, shear and bending of the jets. Observational criteria that can be used to try to distinguish between various possible origins for observed B field structures are described. There is now considerable evidence that the relativistic jets of AGNs do indeed carry helical B fields, with the observed polarization also sometimes appreciably influenced by local effects. Patterns seen in the helical B field components, indicated for example by Faraday rotation observations, provide unique information about the processes involved in the generation of these helical B fields. The collected observations on parsec and kiloparsec scales provide weighty evidence that an important role is played by the action of a cosmic “battery” that acts to generate an inward current along the jet axis and an outward current in a more extended region surrounding the jet.