The jets of AGN as giant coaxial cables

Abstract

Context. The currents carried by the jets of active galactic nuclei (AGNs) can be probed using maps of the Faraday rotation measure (RM), since a jet current will be accompanied by a toroidal magnetic field, which will give rise to a systematic change in the RM across the jet. Aims. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsecscale jets, in order to determine how often helical magnetic fields occur in AGN jets, and to look for overall patterns in the implied directions for the toroidal field components and jet currents. Methods. We have carried out new analyses of Faraday RM maps derived from previously published 8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO Very Long Baseline Array (VLBA). In a number of key ways, our procedures were identical to those of the original authors, but the new imaging and analysis differs from the original methods in several ways: the technique used to match the resolutions at the different frequencies, limits on the widths spanned by the RM gradients analyzed, treatment of core-region RM gradients, approach to estimation of the significances of the gradients analyzed, and inclusion of a supplementary analysis using circular beams with areas equal to those of the corresponding elliptical naturally weighted beams.Results. This new analysis has substantially increased the number of AGNs known to display transverse RM gradients that may reflect the presence of a toroidal magnetic-field component. The collected data on parsec and kiloparsec scales indicate that the current typically flows inward along the jet axis and outward in a more extended region surrounding the jet, typical to the current structure of a co-axial cable, accompanied by a self-consistent system of nested helical magnetic fields, whose toroidal components give rise to the observed transverse Faraday rotation gradients. Conclusions. The new results presented here make it possible for the first time to conclusively demonstrate the existence of a preferred direction for the toroidal magnetic-field components -and therefore of the currents -of AGN jets. Discerning the origin of this current-field system is of cardinal importance for understanding the physical mechanisms leading to the formation of the intrinsic jet magnetic field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a "cosmic battery".

Observational signatures of magnetic field structure in relativistic AGN jets

“…The Article number, page 1 of 14 arXiv:1901.05442v1 [astro-ph.HE] 15 Jan 2019 A&A proofs: manuscript no. main overall picture resembles that of a cosmic co-axial cable with electric current flowing towards the origin of the jet at the inner part of the cable and in the opposite direction in the outer part of the cable (Gabuzda et al 2018).…”

“…The horizontal and vertical axes are scaled so that they they cover the extend of φ d and θ d as described in section 2.1. this twin peak contour structure, whereas some don't (see e.g. Gabuzda et al (2018)). The relatively sharp transition in twisting of the coaxial field leads to a slightly more interesting substructure in the contours of I s , as indicated in Figures 10(a) and (c).…”

“…The left panel is with an elliptical beam (shown at bottom left), the middle panel with a circular beam and the Faraday Rotation measure is shown in the right panel. The projected velocity on the plane of the sky is shown with a red arrow at the middle panel and the slice along which the Faraday Rotation is measures is plotted as a black line in the middle panel.Figure reproducedbyGabuzda et al (2018).…”

“…The authors would like to thank Daniele Dorigoni for a productive discussion regarding the singular value decomposition. We also thank Denise Gabuzda for permission to use Figure 3 from Gabuzda et al (2018) Article number, page 13 of 14 A&A proofs: manuscript no. main and discussion on the choice of the slice along which the RM is measured, and Maxim Lyutikov for insightful comments.…”

Ejection of Double Knots from the Radio Core of PKS 1510–089 during the Strong Gamma-Ray Flares in 2015

“…One of the possible origins of the double knot structure and the corresponding linear polarization structure is a large-scale helical magnetic field permeating in the jet (e.g., Lyutikov et al 2005;Clausen-Brown et al 2011;Murphy et al 2013;Zamaninasab et al 2013). Evidence for helical magnetic fields in the jets of at least some blazars was provided by VLBI observations of Faraday rotation in the jets (e.g., Asada et al 2002;Algaba 2013;Zamaninasab et al 2013;Gómez et al 2016;Gabuzda et al 2018). The helical field, depending on the jet viewing angle and the field pitch angle, can produce asymmetric profiles of both total intensity emission and linear polarization emission transverse to the jet.…”