In the era of the Fermi satellite and of the new generation Cherenkov telescopes, there is an active debate over the location and the mechanisms for the production of MeV to very high energy (VHE) gamma rays in AGN jets. Multi-wavelength polarimetric studies of singular flaring objects can provide a wealth of information about the time scale and correlation of the flaring events at different wavelengths as also physical properties and kinematics of emission features along the jet that could be connected with the gamma-ray emission.
I will present results of our multi-wavelength campaign on the quasar CTA102 that went through an extraordinary bright gamma-ray outburst in late 2012. We analyse data from radio to gamma-rays with a particular focus on the VLBA 43 GHz data. The VLBA data reveal two flares in the radio core, the first one between June and July 2008 and the second one very close to the gamma-ray flare in October 2012. During both flares a new component has been ejected although there are substantial differences between the two: the component ejected during the gamma-ray flare is much weaker and smaller respect to the component resulting in 2008. We analyzed also the polarization of the source at mm and optical wavelengths obtaining a significant increase in the degree of optical polarization during the gamma-rays flare and many fast rotations by optical EVPAs in the clockwise direction. Meanwhile the mm polarization does not show any significant increase during the gamma-ray flare while it displays a peculiar slow rotation in EVPAs during almost three years.
Higher resolution observations at mm wavelength (mm-VLBI) could help to unpuzzle this complicated behaviour. Hence, in the second part of my presentation I focus on the 86 GHz polarimetric observations in the frame of the Global Millimeter VLBI Array (GMVA) project. The reduced opacity at 86 GHz and three times higher resolution, allow us to estimate the angular sizes and magnetic field structure of the most compact features, which can be compared with those observed at 43 GHz with the VLBA for the determination of the jet's physical parameters. I will present recent results from the GMVA observing epoch of May 2016.