Gamma-ray Flaring Emission in Blazar OJ287 Located in the Jet >14 pc from the Black Hole

TL;DR

This study locates gamma-ray emission in the jet of blazar OJ287 more than 14 parsecs from the black hole, using multi-wavelength observations and polarization data to understand flare origins.

Contribution

It provides the first detailed multi-wavelength analysis pinpointing gamma-ray emission sites far from the central engine in a blazar.

Findings

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

Flares originate in jet features separated by over 14 parsecs from the black hole.

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

Abstract

We combine the Fermi-LAT light curve of the BL Lacertae type blazar OJ287 with time-dependent multi-waveband flux and linear polarization observations and submilliarcsecond-scale polarimetric images at lambda=7mm 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 millimeter-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 millimeter-wave flares originated in the second of two features in the jet that are separated by >14pc. 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 millimeter-wave flares are cospatial and occur >14pc 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.