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

TL;DR

This study investigates the polarization and particle acceleration in the jet of OJ 287, revealing that high-energy electrons are accelerated both near the core and at a downstream stationary feature, indicating complex acceleration sites.

Contribution

It provides new evidence that electron acceleration occurs not only at the jet core but also at a downstream stationary shock in OJ 287, using multi-epoch VLBA and optical polarization data.

Findings

Optical and 43 GHz polarization angles match at certain jet locations.

High-energy electrons are accelerated both at the core and downstream feature.

The stationary feature's polarization suggests a conical standing shock.

Abstract

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.