Reversals in the Direction of Polarization Rotation in OJ 287

TL;DR

This study analyzes decades of polarization data from blazar OJ 287, revealing polarization rotation reversals explained by a model involving successive outbursts and a helical magnetic field, supported by polarization imaging.

Contribution

It introduces a model of polarization reversals based on superposed outbursts and a helical magnetic field, explaining both radio and optical polarization events in OJ 287.

Findings

Observed four polarization reversal events over 40 years.

Proposed a two-outburst model with counter-rotating EVPAs.

Supported by polarization imaging and magnetosonic jet theory.

Abstract

We have obtained a smooth time series for the Electric Vector Position Angle (EVPA) of the blazar OJ 287 at centimeter wavelengths, by making $\pm n\pi$ adjustments to archival values from 1974 to 2016. The data display rotation reversals in which the EVPA rotates counter-clockwise (CCW) for 180 deg and then rotates clockwise (CW) by a similar amount. The time scale of the rotations is a few weeks to a year, and the scale for a double rotation, including the reversal, is one to three years. We have seen four of these events in 40 years. A model consisting of two successive outbursts in polarized flux density, with EVPAs counter-rotating, superposed on a steady polarized jet, can explain many of the details of the observations. Polarization images support this interpretation. The model can also help to explain similar events seen at optical wavelengths. The outbursts needed for the model can be generated by the super-magnetosonic jet model of Nakamura et al. (2010) and Nakamura and Meier (2014), which requires a strong helical magnetic field. This model produces forward and reverse pairs of fast and slow MHD waves, and the plasma inside the two fast/slow pairs rotates around the jet axis, but in opposite directions.