Stratification in polarization and Faraday rotation in the jet of 3C 120

TL;DR

This study uses VLBA polarimetric observations to reveal stratified Faraday rotation and polarization in the jet of 3C 120, suggesting a helical magnetic field and jet-medium interaction.

Contribution

It provides detailed evidence of polarization stratification and Faraday rotation gradients, supporting a two-fluid jet model with a helical magnetic field in 3C 120.

Findings

Polarization is asymmetric across the jet width.

Faraday rotation measure varies across the jet, with higher values on the southern side.

A localized high Faraday rotation region suggests jet-medium interaction.

Abstract

Very long baseline interferometric observations of the radio galaxy 3C 120 show a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. These are revealed by the passage of multiple superluminal components throughout the jet as they move out from the core in a sequence of 12 monthly polarimetric observations taken with the VLBA at 15, 22, and 43 GHz. The degree of polarization has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. Superposed on this structure we find a localized region of high Faraday rotation measure (about 6000 rad/m^2) between approximately 3 and 4 mas from the core. This region of enhanced Faraday rotation may result from the interaction of the jet with the ambient medium, which may also explain the stratification in degree of polarization. The data are also consistent with a helical magnetic field in a two-fluid jet model, consisting of an inner emitting jet and a sheath of nonrelativistic electrons.