The Physics of Blazar Optical Emission Regions I: Alignment of Optical   Polarization and the VLBI Jet
  We collected optical and near IR linear polarization data obtained over 20--30 years for a sample of 51 blazars. For each object, we calculated the probability that the distribution of position angles was isotropic. The distribution of these probabilities was sharply peaked, with 27 blazars showing a probability < 15% of an isotropic distribution of position angles.   For these 27 objects we defined a preferred position angle. For those 17 out of 27 blazars showing a well-defined radio structure angle (jet position angle) on VLBI scales (1--3mas), we looked at the distribution of angle differences -- the optical polarization relative to the radio position angles. This distribution is sharply peaked, especially for the BL Lac objects, with alignment better than 15 degrees for half the sample. Those blazars with preferred optical position angles were much less likely to have bent jets on 1--20mas scales. These results support a shock-in-jet hypothesis for the jet optical emission regions. 
Michael J. Yuan, Hien Tran, Beverley Wills, D. Wills