Optical Emission and Particle Acceleration in a Quasi-Stationary Component in the Jet of OJ~287

Mahito Sasada, Svetlana Jorstad, Alan P. Marscher, Vishal Bala, Manasvita Joshi, Nicholas R. MacDonald, Michael P. Malmrose, Valeri M. Larionov, Daria A. Morozova, Ivan S. Troitsky, Iván Agudo, Carolina Casadio, José L. Gómez, Sol N. Molina, Ryosuke Itoh

Published 2018-07-30Version 1

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.