Location of gamma-ray Flare Emission in the Jet of the BL Lacertae Object OJ287 more than 14pc from the Central Engine

TL;DR

This study locates gamma-ray emission in the jet of blazar OJ287 more than 14 parsecs from its central engine by analyzing multi-waveband observations and polarimetric imaging, revealing co-spatial flares and their physical mechanisms.

Contribution

It provides the first direct evidence that gamma-ray flares originate more than 14 parsecs from the central engine in OJ287, using combined multi-waveband and VLBA polarimetric data.

Findings

Gamma-ray and millimeter-wave flares are highly correlated.

Flares originate in jet features >14 pc from the central engine.

Gamma-ray emission is likely due to synchrotron self-Compton scattering.

Abstract

We combine time-dependent multi-waveband flux and linear polarization observations with sub-milliarcsecond-scale polarimetric images at lambda=7mm of the BL Lacertae-type blazar OJ287 to locate the gamma-ray emission in prominent flares in the jet of the source >14pc from the central engine. We demonstrate a highly significant correlation between the strongest gamma-ray and millimeter-wave flares through Monte-Carlo simulations. The two reported gamma-ray peaks occurred near the beginning of two major mm-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our Very Long Baseline Array observations indicate that the two mm-wave flares originated in the second of two features in the jet that are separated by >14 pc. The simultaneity of the peak of the higher-amplitude gamma-ray flare and the maximum in polarization of the second jet feature implies that the gamma-ray and mm-wave flares are co-spatial and occur >14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two gamma-ray events. The multi-waveband behavior is most easily explained if the gamma-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The gamma-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The mm-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.