Parsec scale Faraday-rotation structure across the jets of nine active galactic nuclei

Abstract

A number of groups have recently been active in searching for gradients in the observed Faraday rotation measure (RM) across the jets of active galactic nuclei (AGNs) on various scales and estimating their reliability. Such RM structures provide direct evidence for the presence of an azimuthal magnetic field component, which may be associated with a helical jet magnetic field, as is expected based on the results of many theoretical studies. We present new parsec-scale RM maps of four AGNs here, and analyse their transverse RM structures together with those for five previously published RM maps. All these jets display transverse RM gradients with significances of at least 3σ. This is part of an ongoing effort to establish how common transverse RMgradients that may be associated with helical or toroidal magnetic fields are in AGNs on parsec scales.