Faraday rotation in the MOJAVE blazars: 3C 273 a case study
  Radio polarimetric observations of Active Galactic Nuclei can reveal the magnetic field structure in the parsec-scale jets of these sources. We have observed the gamma-ray blazar 3C 273 as part of our multi-frequency survey with the Very Long Baseline Array to study Faraday rotation in a large sample of jets. Our observations re-confirm the transverse rotation measure gradient in 3C 273. For the first time the gradient is seen to cross zero which is further indication for a helical magnetic field and spine-sheath structure in the jet. We believe the difference to previous epochs is due to a different part of the jet being illuminated in our observations. 
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 astro-ph.CO