Evidence for Helical Magnetic Fields Associated with AGN Jets and the Action of a Cosmic Battery

Abstract

Theoretical models for the electromagnetic launching of astrophysical jets have long indicated that this process should generate helical magnetic fields, which should then propagate outward with the jet plasma. Polarization observations of jets are key for testing this idea, since they provide direct information about the magnetic field structures in the synchrotron-emitting radio jets. Together with Faraday rotation measurements, it is possible in some cases to reconstruct the three-dimensional magnetic-field structure. There is now plentiful evidence for the presence of helical magnetic fields associated with the jets of active galactic nuclei, most directly the detection of transverse Faraday-rotation gradients indicating a systematic change in the line-of-sight magnetic field component across the jets. A variety of models involving helical jet magnetic fields have also been used to explain a great diversity of phenomena, including not only the linear polarization and Faraday rotation structures, but also circular polarization, anomalous wavelength dependences of the linear polarization, variability of jet ridge lines, variability of the Faraday rotation sign and polarization angle rotations. A joint consideration of Faraday rotation measurements on parsec and kiloparsec scales indicates a magnetic-field and current structure similar to that of a co-axial cable, suggesting the action of some kind of battery mechanism, such as the Poynting–Robertson cosmic battery.

Multiwavelength Variability of BL Lacertae Measured with High Time Resolution

“…This can be reconciled by the existence of partial ordering of the magnetic field. For example, the partial ordering along the jet direction inferred from the mean EVPA can be the consequence of compression of the turbulent plasma by a shock (e.g., Hughes et al 1985;Cawthorne 2006;Marscher 2014) or the superposition of turbulence and a helical magnetic field (e.g., Raiteri et al 2010;Gabuzda 2018).…”

“…Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research made use of Astropy, 8 a community-developed core Python package for Astronomy (Astropy Collaboration et al 2013, 2018, and astroquery, an astronomical web-querying package in Python (Ginsburg et al 2019).…”

Rapid particle acceleration due to recollimation shocks and turbulent magnetic fields in injected jets with helical magnetic fields

“…Studies in the slab configuration have shown significant particle acceleration. However, the slab configuration cannot be applied to astrophysical jets, since helical magnetic fields are thought to be the dominant magnetic field morphology close to the jet collimation/launching point (e.g., Tchekhovskoy 2015), as evidenced by observations (e.g., Hawley et al 2015;Gabuzda 2019).…”

AGN Singularities and Jets Modelled with the Superstar Scenario

“…The collected data indicate that AGN jets reveal strong signs of the spine-sheath jet structure with a central spine of orthogonal magnetic field and a sheath of longitudinal magnetic field along the edges of the jet [26]. They suggest that the inner region of the jet spine consists of a low-density and fast-moving gas, while the outer region of the jet sheath consists of a denser and slower moving gas [27] The spine-sheath jet structure can also be explained by the coaxial cable proposed by Gabuzda [28] [29]. Like coaxial cable with the inner conductor and the outer shield sharing a geometric axis, an AGN jet has an inward current along its axis and an outward current in a more extended sheath-like region surrounding the jet.…”