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