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J. Yuan (U. Texas at Austin), H. D. Tran (IGPP, LLNL), B.J. Wills, D. Wills (U. Texas at Austin)
We collected optical and IR linear polarization data observed over a time span of 20--30 years for a sample of 53 blazars in order to investigate secular trends and compare these with radio jet parameters. We included data from the literature as well as unpublished measurements from McDonald Observatory. Radio structure data were from the literature. We used VLBI measurements where available, otherwise VLA maps.
Most blazars did not show significant secular trends. We used circular statistics to parameterize the polarization and its variability. We found:
1). 20 BL Lac objects and 9 highly polarized quasars individually had greater than 85% probability of a non-random (preferred) polarization position angle.
2). Preferred polarization position angles were aligned with the VLBI (or VLA) radio structures for BL Lac objects. For QSOs, the two angles seemed independent. We did not find any tendency for polarization position angles to be perpendicular to the radio axes.
3). There is a positive correlation between the circular standard deviation of polarization position angle and radio core dominance log R (significant at the 2.5% and 0.05% levels for QSOs and BL Lac objects.)
4). We did not find any correlation between the percentage polarization and log R.
We will discuss our results in terms of the physics and models of blazars.