2016A&A...586A..60K

Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS1502+106, which showed a rapid and strong γ-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign that also covered the radio, optical, UV, and X-ray bands, indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands. We explore the phenomenology and physical conditions within the ultra-relativistic jet of the γ-ray blazar PKS1502+106. Additionally, we address the question of the spatial localization of the MeV/GeV-emitting region of the source. We used ultra-high angular resolution mm-VLBI observations at 43 and 86GHz complemented by VLBI observations at 15GHz. We also employed single-dish radio data from the F-GAMMA program at frequencies matching the VLBI monitoring. PKS1502+106 shows a compact core-jet morphology and fast superluminal motion with apparent speeds in the range 5-22c. Estimating Doppler factors along the jet yields values of between ∼7 up to ∼50. This Doppler factor gradient implies an accelerating jet. The viewing angle towards the source differs between the inner and outer jet, with the former at θ∼3° and the latter at θ∼1°, after the jet bends towards the observer beyond 1mas. The de-projected opening angle of the ultra-fast magnetically dominated jet is found to be (3.8±0.5)°. A single jet component can be associated with the pronounced flare both at high energies and in radio bands. Finally, the γ-ray emission region is localized at ≤5.9pc away from the jet base.