Synchronous Optical and Radio Polarization Variability in the Blazar
  OJ287
  We explore the variability and cross-frequency correlation of the flux
density and polarization of the blazar OJ287, using imaging at 43 GHz with the
Very Long Baseline Array, as well as optical and near-infrared polarimetry. The
polarization and flux density in both the optical waveband and the 43 GHz
compact core increased by a small amount in late 2005, and increased
significantly along with the near-IR polarization and flux density over the
course of 10 days in early 2006. Furthermore, the values of the electric vector
position angle (EVPA) at the three wavebands are similar. At 43 GHz, the EVPA
of the blazar core is perpendicular to the flow of the jet, while the EVPAs of
emerging superluminal knots are aligned parallel to the jet axis. The core
polarization is that expected if shear aligns the magnetic field at the
boundary between flows of disparate velocities within the jet. Using variations
in flux density, percentage polarization, and EVPA, we model the inner jet as a
spine-sheath system. The model jet contains a turbulent spine of half-width 1.2
degrees and maximum Lorentz factor of 16.5, a turbulent sheath with Lorentz
factor of 5, and a boundary region of sheared field between the spine and
sheath. Transverse shocks propagating along the fast, turbulent spine can
explain the superluminal knots. The observed flux density and polarization
variations are then compatible with changes in the direction of the inner jet
caused by a temporary change in the position of the core if the spine contains
wiggles owing to an instability. In addition, we can explain a stable offset of
optical and near-IR percentage polarization by a steepening of spectral index
with frequency, as supported by the data.
