出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：To impose some kind of order on a broad review of various recent observational VLBI polarisation
results, these results will be considered in the framework of the hypothesis that many Active
Galactic Nuclei (AGN) jets have helical magnetic (B) fields. The results of the MOJAVE project,
in which 133 radio-bright AGN are regularly monitored at 15 GHz with polarisation sensitivity
with the VLBA, represents an extremely useful resource that is now available to the astronomical
community. There are a growing number of AGN whose jets show clear evidence for the presence
of helical B fields: extended regions in the VLBI jets where the predominant B field is orthogonal
to the jet direction, stretches of VLBI jets where the inferred B field remains nearly orthogonal
to the jet direction in the presence of substantial bending of the jet, the common occurrence of
“spine+sheath” jet polarisation structures and the presence of Faraday-rotation gradients across
the VLBI jets. This compels us to take very seriously the idea that some, possibly many, AGN
jets have helical B fields. In fact, such fields can come about very naturally as a consequence
of the joint action of rotation of the central black hole+accretion disc and the jet outflow. As a
whole, the recent observational results considered here suggest that we must look at AGN jets as
fundamentally electromagnetic, current-carrying structures if we wish to fully understand their
nature. Possible observational tests to distinguish between alternative interpretations for some of
the observed phenomena are also discussed.
