Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of 37 gamma-ray bright blazars at optical and - with VLBA imaging at a resolution near 0.1 milliarcseconds - millimeter wavelengths. The time-dependent behavior includes both patterns, such as rotations of the polarization vector and fluctuations about "preferred" position angles, and apparent randomness. This implies the presence of ordered structures - very narrow jets and both standing and moving shocks - as well as a stochastic process such as turbulence. The lead author's Turbulent Extreme Multi-Zone (TEMZ) model reproduces the general nature of the observations if the efficiency of particle acceleration depends on magnetic field direction relative to shock fronts. We have started to produce circularly polarized intensity VLBA images at 7 mm of a sub-sample with high signal-to-noise. The offsets of the locations of peaks of the total, linearly polarized, and circularly polarized images discriminate among specific models with different levels of ordered relative to disordered magnetic field and ratios of positrons to protons in the jet. This research is funded in part by US National Science Foundation grant AST-1615796, and by NASA through Fermi grant 80NSSC17K0649 and Swift grant 80NSSC17K0309.
American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020