LOCATION OF γ-RAY FLARE EMISSION IN THE JET OF THE BL LACERTAE OBJECT OJ287 MORE THAN 14 pc FROM THE CENTRAL ENGINE

Abstract

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