The parsec-scale Faraday rotation measure ( RM ) distribution of six “ blazars ” ( 0954+658 , 1156+295 , 1418+546 , 1749+096 , 2007+777 & 2200+420 ) is investigated using multi-frequency ( 4.6 , 5.1 , 7.9 , 8.9 , 12.9 , 15.4 , 22 & 43 GHz ) polarization observations taken on 2006 July 2 with the American Very Long Baseline Array ( VLBA ) .
Analysis of the RM provides the direction of the line-of-sight ( LoS ) magnetic field component , as well as the intrinsic ( unrotated ) 2-D 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 } } | \propto \nu ^ { 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 .