Connecting Magnetic Towers with Faraday Rotation Gradients in Active
  Galactic Nuclei Jets
  The idea that systematic Faraday Rotation gradients across the parsec-scale
jets of AGNs can reveal the presence of helical magnetic (B) fields has been
around since the early 1990s. These gradients are taken to be due to the
systematic variation of the line of sight-B-field across the jet. We present
here the parsec-scale Faraday Rotation distributions for the BL Lac objects
0716+714 and 1749+701, based on polarization data obtained with the Very Long
Baseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cm
bands (0716+714) and at four wavelengths in the range 18-22 cm (1749+701). The
Rotation Measure (RM) maps for both these sources indicate systematic gradients
across their jets, as expected if these jets have helical B fields. The
significance of these transverse RM gradients is > 3 sigma in all cases. We
present the results of Monte Carlo simulations directly demonstrating the
possibility of observing such transverse RM gradients even if the intrinsic jet
structure is much narrower than the observing beam. We observe an intriguing
new feature in these sources, a reversal in the direction of the gradient in
the jet as compared to the gradient in the core region. This provides new
evidence to support models in which field lines emerging from the central
region of the accretion disk and closing in the outer region of the accretion
disk are both "wound up" by the differential rotation of the disk. The net
observed RM gradient will essentially be the sum effect of two regions of
helical field, one nested inside the other. The direction of the net RM
gradient will be determined by whether the inner or outer helix dominates the
RM integrated through the jet, and RM gradient reversals will be observed if
the inner and outer helical fields dominate in different regions of the jet.
This potentially provides new insights about the overall configuration of the
jet B fields.
