Toroidal magnetic fields and associated currents in AGN jets on kiloparsec scales
Helical fields are generally accepted as the magnetic field configuration associated with the launching of astrophysical jets, but it is unclear what role they play on the largest jet scales (if they persist to such distances from the central active galactic nucleus (AGN) ). The synchrotron emission from these jets can be highly linearly polarized, making the behaviour of the polarized radiation a useful tool in examining the magnetic fields of these jets. Observations at multiple wavelengths can be used to construct Faraday rotation measure (RM) images, which provide information about the line-of-sight magnetic field. A significant gradient in the RM transverse to the jet direction may indicate a toroidal or helical magnetic field, which would, in turn, imply the presence of an associated electrical current in the jet. The detection of such gradients on kiloparsec scales can reliably demonstrate that helical or toroidal fields can persist to large distances from the central AGN. We present kiloparsec-scale Faraday rotation maps for NGC$~$6251, 3C$~$303 and 3C$~$219 based on archival data from the Very Large Array. NGC$~$6251 shows statistically significant transverse RM gradients across its kiloparsec scale jet structure. A previous report of a transverse RM gradient across the jet of 3C$~$303 proves not to be significant, and we have identified a tentative transverse RM gradient across the jet of 3C$~$219. The NGC$~$6251 gradient adds to previous detections of significant transverse Faraday rotation gradients across large-scale jets, which have all corresponded to outward currents. Together with results for AGN jets on parsec scales, which imply inward currents, this suggests that the jets have a current/magnetic field configuration similar to that of a co-axial cable.