2013MNRAS.431..695M

The idea that systematic Faraday Rotation gradients across the parsec-scale jets of active galactic nuclei (AGNs) can reveal the presence of helical magnetic (\boldsymbol B) fields has been around since the early 1990s, although the first observation of this phenomenon was about ten years later. These gradients are taken to be due to the systematic variation of the line-of-sight \boldsymbol 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 at two wavelengths near each of the 2cm, 4cm and 6cm bands (0716+714) and at four wavelengths in the range 18-22cm (1749+701). The Rotation Measure (RM) maps for both these sources indicate systematic gradients across their jets, as expected if these jets have helical \boldsymbol B fields. The significance of these transverse RM gradients is >3σ 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 disc and closing in the outer region of the accretion disc are both `wound up' by the differential rotation of the disc. 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 \boldsymbol B fields.