Faraday rotation in the MOJAVE blazars: Connection with gamma-ray   studies
  We have conducted a survey of Faraday rotation in a sample of 191 compact radio-loud AGNs as part of the MOJAVE (Monitoring of Jets in Active galactic nuclei with VLBA Experiments) project. The observations were carried out with the VLBA at 8.1, 8.4, 12.1 and 15.3 GHz over 12 epochs in 2006. We detect sufficiently strong linear polarization in 159 out of 211 observations to calculate the rotation measure values, resulting in a large enough sample for statistical analysis of the Faraday rotation in blazars. These Faraday rotation measures can be used to study the intrinsic magnetic field order and orientation in parsec-scale blazar jets. Our sample includes 119 sources listed in the 1FGL or 2FGL catalogs and we detect rotation measure values in 111 out of 131 maps. Of the 72 sources that are not in the gamma-ray catalogs we detect RM in 48 out of 80 maps. The median RM values of the LAT-detected sources do not differ significantly from the non-LAT-detected sources. Nine of the sources in our sample have resolved enough jets to study the transverse Faraday rotation structure, and we detect significant transverse rotation measure gradients in four sources. In two of these (3C~273 and 3C~454.3) there is additional evidence to support helical magnetic field in the parsec-scale jets. The two others (0923+392 and 2230+114) require further observations to identify the nature of the gradient. It is interesting that three of the four sources with significant rotation measure gradients are sources that have shown large gamma-ray flares. 
T. Hovatta, M. L. Lister, M. F. Aller, H. D. Aller, D. C. Homan, Y. Y.   Kovalev, A. B. Pushkarev and T. Savolainen
astro-ph.HE