Abstract
Though helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute substantially to the collimation of the jet. This B-field structure can be detected in images of the Faraday rotation measure (RM)—a measure of the change in polarisation angle of an electromagnetic wave as it passes through a magneto-ionic medium. The Faraday rotation measure is directly proportional to the line-of-sight magnetic field; therefore a monotonic gradient in the RM transverse to the jet indicates similar behaviour of the line-of-sight B-field component. This type of analysis has mostly been done on parsec scales using VLBI observations at centimetre wavelengths, while relatively few studies have probed decaparsec to kiloparsec scales. The detection of RM gradients with significances of 3 σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN. We present the results of new Faraday rotation analyses for 2 AGN on kiloparsec scales based on multiwavelength VLA observations, with robust transverse RM gradients detected in both. Furthermore, the direction of the inferred toroidal B-fields on the sky supports previous results indicating a predominance of outward currents in the jets on kiloparsec scales.
Highlights
Analysis of the polarized emission from radio jets from Active Galactic Nuclei (AGN) is a powerful tool for investigating their magnetic (B) field structure
The detection of rotation measure (RM) gradients with significances of 3σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN
The sign of the RM changes along these transverse RM gradients, which can only be explained by a change in direction of the magnetic field along the line of sight and not by a change in electron density
Summary
Analysis of the polarized emission from radio jets from Active Galactic Nuclei (AGN) is a powerful tool for investigating their magnetic (B) field structure. B-field can be investigated using Faraday rotation, the change in the angle of linear polarization as the electromagnetic wave passes through a region with free electrons and magnetic field. A gradient in the RM transverse to the jet indicates a similar trend in the line-of-sight magnetic field. Such a gradient is a strong indicator for a toroidal magnetic field component, which may be confining the jet. The significance of an RM gradient was calculated using its end point values and associated errors This is a conservative estimate as the error in the RM increases at the edges of the jet as there is less polarized intensity. The significance was calculated by dividing the total change in the RM by the errors at the end points of the gradients added in quadrature
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