Three-dimensional magnetic field structure of six parsec-scale active galactic nuclei jets

Abstract

The parsec-scale Faraday rotation measure (RM) distribution of six 'blazars' (0954+658, 1156+295, 1418+546, 1749+096, 2007+777 and 2200+420) are investigated using multifrequency (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22 and 43 GHz) polarization observations with the American Very Long Baseline Array taken on 2006 July 2. Analysis of the RM provides the direction of the line-of-sight (LoS) magnetic field component, as well as the intrinsic (unrotated) two-dimensional polarization distribution on the plane of the sky. Our results show that the magnitude of the core RM increases systematically with frequency, and is well described by a power law, where |RM core | ∝ v a . Our measured values of a vary from 0.9 to 3.8, providing information on the assumed power-law fall-off in the electron density (n e ) with distance from the central engine (r) for each source. RM gradients were detected across the jets of 0954+658, 1156+295 and 1418+546, supporting the presence of helical magnetic fields in a sheath or boundary layer surrounding their jets. We find a bi-modal distribution of the intrinsic jet polarization orientation, with the polarization angles either aligned or orthogonal to the jet direction. The polarization of 2200+420 displays a continuous structure, with the polarization angles remaining aligned with the jet direction even as it bends. This indicates that the magnetic field structure in the synchrotron emitting plasma is dominated by an ordered transverse component. A helical magnetic field geometry can neatly explain both the bi-model distribution of the jet polarization orientation and the ordered polarization structure on these scales. For 0954+658, 1418+546 and 2200+420, we find that the core RM changes sign with distance from the central engine. We provide an explanation for this by considering a boundary layer of Faraday rotating material threaded by a helical magnetic field, where bends in the relativistic jet or accelerating/decelerating flows give rise to changes in the dominant LoS components of the magnetic field, which in turn gives rise to different signs of the RM.