Are There Rotation Measure Gradients Across AGN Jets?
  We report on multi-frequency polarimetry Very Long Baseline Interferometry
observations of active galactic nuclei using the VLBA. These observations are
used to construct images of the Faraday Rotation Measure (RM) in J1613+342, Mrk
501, 3C 371, and BL Lac. Despite having resolved the jets in total intensity
and polarization for three of these sources no RM gradients are found. This is
in contrast to the large fraction of sources with RM gradients now claimed in
the literature, and invoked as evidence in support of helical magnetic fields.
We propose objective criteria for establishing what constitutes an RM gradient.
Furthermore, although we note the absence of simple, monotonic gradients,
comparison with simulations could reveal systematic changes in the RM which may
be masked by a varying jet orientation.
