A shocking outcome: Jet dynamics and polarimetric signatures of the multi-band flare in blazar OJ 248

TL;DR

This study investigates the connection between a gamma-ray flare and jet structure in blazar OJ 248, revealing polarimetric changes consistent with a shock interaction that produces the flare.

Contribution

It presents the first detailed analysis linking gamma-ray flares to jet polarization and morphology changes in OJ 248, proposing a shock-shock interaction model.

Findings

Increased linear polarization during the flare

EVPA rotations perpendicular to jet flow

Evidence for a moving shock interacting with a recollimation shock

Abstract

The connection between $\gamma$-ray flares and blazars is a topic of active research, with few sources exhibiting distinct enough such outbursts to be able to conclusively connect them to features in their jet morphology. Here we present an investigation of the sole $\gamma$-ray flare of the blazar OJ 248 thus far, in association with its jet structure, as revealed by very long baseline interferometry (VLBI). We find that throughout the course of the $\gamma$-ray flare, the fractional linear polarisation increases in the jet of OJ 248, and the VLBI electric vector position angles (EVPAs) turn perpendicular to the bulk jet flow. We interpret this behaviour as a moving shock, travelling through a recollimation shock and upscattering photons via the inverse Compton scattering process, producing a $\gamma$-ray flare; we discuss possible mechanisms. Our hypothesised shock-shock interaction scenario is a viable mechanism to induce such EVPA rotations in both optical and radio bands.