Optical Emission and Particle Acceleration in a Quasi-Stationary
  Component in the Jet of OJ~287
  We analyze the linear polarization of the relativistic jet in BL Lacertae
object OJ~287 as revealed by multi-epoch Very Long Baseline Array (VLBA) images
at 43 GHz and monitoring observations at optical bands. The electric-vector
position angle (EVPA) of the optical polarization matches that at 43 GHz at
locations that are often in the compact millimeter-wave "core" or, at other
epochs, coincident with a bright, quasi-stationary emission feature
$\sim0.2$~milliarcsec ($\sim$0.9~pc projected on the sky) downstream from the
core. This implies that electrons with high enough energies to emit optical
synchrotron and $\gamma$-ray inverse Compton radiation are accelerated both in
the core and at the downstream feature, the latter of which lies $\geq10$~pc
from the central engine. The polarization vector in the stationary feature is
nearly parallel to the jet axis, as expected for a conical standing shock
capable of accelerating electrons to GeV energies.
