γ-ray emission region located in the parsec scale jet of OJ287

Abstract

We report on the location of the γ-ray emission region in flares of the BL Lacertae object OJ287 at > 14 pc from the central supermassive black hole. We employ data from multi-spectral range (total flux and linear polarization) monitoring programs combined with sequences of ultra-high-resolution 7 mm VLBA images. The correlation between the brightest γ-ray and mm flares is found to be statistically significant. The two γ-ray peaks, detected by Fermi-LAT, that we report here happened at the rising phase of two exceptionally bright mm flares accompanied by sharp linear polarization peaks. The VLBA images show that these mm flares in total flux and polarization degree occurred in a jet region at > 14 pc from the innermost jet region. The time coincidence of the brighter γ-ray flare and its corresponding mm linear polarization peak evidences that both the γ-ray and mm outbursts occur > 14 pc from the central black hole. We find two sharp optical flares occurring at the peak times of the two reported γ-ray flares. This is interpreted as the γ-ray flares being produced by synchrotron self-Compton scattering of optical photons from the flares triggered by the interaction of moving knots with a stationary conical shock in the jet.