Surprising Evolution of the Parsec-scale Faraday Rotation Gradients in
  the Jet of the BL Lac Object B1803+784
  Several multi-frequency polarization studies have shown the presence of
systematic Faraday Rotation gradients across the parsec-scale jets of Active
Galactic Nuclei (AGN), taken to be due to the systematic variation of the
line-of-sight component of a helical magnetic (B) field across the jet. Other
studies have confirmed the presence and sense of these gradients in several
sources, thus providing evidence that these gradients persist over time and
over large distances from the core. However, we find surprising new evidence
for a reversal in the direction of the Faraday Rotation gradient across the jet
of B1803+784, for which multi-frequency polarization observations are available
at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we
observe transverse Rotation Measure (RM) gradients across the jet, consistent
with the presence of a helical magnetic field wrapped around the jet. However,
we also observe a "flip" in the direction of the gradient between June 2000 and
August 2002. Although the origins of this phenomena are not entirely clear,
possibly explanations include (i) the sense of rotation of the central
supermassive black hole and accretion disc has remained the same, but the
dominant magnetic pole facing the Earth has changed from North to South; (ii) a
change in the direction of the azimuthal B field component as a result of
torsional oscillations of the jet; and (iii) a change in the relative
contributions to the observed rotation measures of the "inner" and "outer"
helical fields in a magnetic-tower model. Although we cannot entirely rule out
the possibility that the observed changes in the RM distribution are associated
instead with changes in the thermal-electron distribution in the vicinity of
the jet, we argue that this explanation is unlikely.
