Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?

TL;DR

This study uses multi-frequency VLBA polarimetric observations of 3C 120 to analyze magnetic field structures and jet-medium interactions, revealing polarization gradients, stratified Faraday rotation, and evidence for a helical magnetic field with localized Faraday rotation enhancements.

Contribution

It provides new insights into the magnetic field configuration and external medium interaction in the jet of 3C 120 through detailed polarization and Faraday rotation analysis.

Findings

Polarization increases with distance from the core and toward jet edges.

Faraday rotation measure is stratified across the jet width.

Localized high Faraday rotation suggests jet-medium interaction.

Abstract

We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA observations of the radio galaxy 3C 120 revealing a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. The degree of polarization increases with distance from the core and toward the jet edges, and 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. We find a localized region of high Faraday rotation measure superposed on this structure between approximately 3 and 4 mas from the core, with a peak of about 6000 rad/m^2. Interaction of the jet with the external medium or a cloud would explain the confined region of enhanced Faraday rotation, as well as the stratification in degree of polarization and the flaring of superluminal knots when crossing this region. The data are also consistent with a helical field in a two-fluid jet model, consisting of an inner, emitting jet and a sheath containing nonrelativistic electrons. However, this helical magnetic field model cannot by itself explain the localized region of enhanced Faraday rotation. The polarization electric vectors, predominantly perpendicular to the jet axis once corrected for Faraday rotation, require a dominant component parallel to the jet axis (in the frame of the emitting plasma) for the magnetic field in the emitting region.