|Denise Gabuzda (University College Cork)
| Standard theoretical models for the formation and launching of the relativistic jets of Active Galactic Nuclei (AGNs) predict the development of a helical magnetic-field component, due to the combination of the rotation of the central supermassive black hole and its accretion disk and the jet outflow.
One elegant way to detect such helical magnetic fields observationally is through Faraday rotation images of the jets: since the Faraday rotation depends on the line-of-sight magnetic-field component, the presence of a helical field component should give rise to a systematic gradient in the observed Faraday rotation across the jet. Furthermore, Monte Carlo simulations have demonstrated that this effect is detectable even across very narrow jets.
Monotonic transverse Faraday-rotation gradients have now been observed across the parsec-scale (VLBI) jets of 27 AGNs, providing firm evidence that they carry helical magnetic fields. Reversals in the direction of these transverse Faraday-rotation gradients that have been observed in several jets provide the first observational evidence for the 'return field', which forms a nested-helical-field structure together with the 'outgoing' field. Intriguing asymmetry in the orientations of the 27 observed transverse Faraday rotation gradients suggests that the direction of the longitudinal field component and the direction of the rotation of the central accretion disk are coupled.