Evidence for a Toroidal Magnetic-Field Component in 5C4.114 on Kiloparsec Scales

TL;DR

This study presents evidence of a toroidal magnetic field component in the AGN 5C4.114 on kiloparsec scales, inferred from significant transverse RM gradients, indicating the persistence of helical magnetic fields far from the central black hole.

Contribution

The paper provides the first kiloparsec-scale evidence of a helical magnetic field in an AGN jet, extending previous parsec-scale findings and demonstrating the survival of such fields over large distances.

Findings

Detected a 4sigma transverse RM gradient across the northern lobe.

Identified a 3sigma RM gradient across the southern jet/lobe.

Observed patchy RM distribution indicating foreground and local contributions.

Abstract

A monotonic, statistically significant gradient in the observed Faraday Rotation Measure (RM) across the jet of an Active Galactic Nucleus (AGN) reflects a corresponding gradient in the electron density and/or line-of-sight magnetic (B) field. Such gradients may indicate the presence of a toroidal B field component, possibly associated with a helical jet B field. Although transverse RM gradients have been reported across a number of parsec-scale AGN jets, the same is not true on kiloparsec scales, suggesting that other (e.g. random) B-field components usually dominate on these larger scales. We have identified an extended, monotonic transverse RM gradient across the Northern lobe of a previously published Very Large Array (kiloparsec-scale) RM image of 5C4.114. We reanalyzed these VLA data in order to determine the significance of this RM gradient. The RM gradient across the Northern kiloparsec-scale lobe structure of 5C4.114 has a statistical significance of about 4sigma. There is also a somewhat less prominent monotonic transverse RM gradient across the Southern jet/lobe (significance ~ 3sigma). Other parts of the RM distribution observed across the source are patchy and show no obvious order.This suggests that we are observing a random RM component associated with the foreground material in the cluster in which the radio source is located and through which it is viewed, superposed on a more ordered RM component that arises in the immediate vicinity of the AGN jets. We interpret the transverse RM gradient as reflecting the systematic variations of the line-of-sight component of a helical or toroidal B field associated with the jets of 5C4.114. These results suggest that the helical field that arises due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of thousands of parsec from the central engine.