Rotation measure gradient in 3C 216
Left: VLBA rotation measure (RM) map of 3C 216 from 12 to 24 GHz. A clear gradient transverse to the direction of the jet is observed. Thick line indicates the region where slice is taken. Right: Slice of the RM and fractional polarization. Beamsize is indicated by the horizontal line in the bottom left. Fractional polarization shows indications of an increase towards the edges of the jet.
Magnetic fields along AGN jets are expected to have a helical morphology due to frame dragging in the vicinity of the SMBH. A way to study the structure of the intrinsic magnetic field is the analysis of the Faraday rotation measure (RM) distribution across the source. The RM depends on the component of the magnetic field along to the line of sight. Hence, if the jet contains a helical magnetic field, its toroidal component will produce a gradient, and possibly a sign reversal, on the RM across the jet. This seems to be indeed the case for the QSO 3C 216, which shows a clear transverse gradient of the rotation measure spanning downstream the jet along 2 mas (~14 pc). The degree of polarization, increasing towards the edges also supports this idea.