Agudo, Iván

The "Far Distance" Scenario for Gamma-ray Emission in Blazars: A View from the VLBI Observing Perspective. 

Since the birth of gamma-ray astronomy, locating the origin of gamma-ray emission has been a fundamental problem for the knowledge of the emission processes involved. Deep and densely time sampled monitoring programs with the Very Long Baseline Array and the Fermi Gamma-ray Space Telescope, together with several other facilities at most of the available spectral ranges (including polarization measurements if possible) are starting to shed light for the case of blazars. A successful observing technique consists on analyzing the timing of multi-waveband variations in the flux and linear polarization, as well as changes in ultra-high resolution VLBI images to associate the particularly bright events at different wavebands. Such association can be robustly demonstrated by probing the statistical significance of the correlation among spectral ranges through Monte Carlo simulations. The location of the high energy emission region is inferred through its relative location with regard to the associated low energy event observed in the VLBI images. In this talk I will present some of the latest results obtained in the wake of our group that locate the GeV emission within the jets of blazars AO 0235+164 and OJ287 at >12 parsec from the central AGN engine, hence supporting the "Far Distance" scenario.