Studies of non-thermal emission from AGN through polarization observations at the VLT

In order to constrain models of particle acceleration in Active Galaxies with spectral energy distributions (SEDs) that are dominated by non-thermal radiation mechanisms, we carried out polarimetric observations with the FORS1 instrument on UT1 of the ESO-VLT. These studies involve imaging and spectropolarimetry of both circular and linear modes of polarization. Firstly, we studied the optical polarization from hot spots. For the first time the polarization could be studied in spatially extended regions. Because of the short radiative lifetimes of electrons emitting optical synchrotron emission, this is the first demonstration that particle acceleration must occur in situ over a spatial volume much larger than the hot spot proper. An attractive explanation is acceleration by shocks caused by the backflows of the propagating heads of the plasma jets. Confirmation of this suggestion would open an entirely new approach to studies of the jet-IGM interaction. A second project are studies of the circular polarization of AGN with very high brightness temperatures. This is a unique way of investigating the role of collective acceleration of particles and coherent synchrotron radiation. This is also very challenging a very from a technical point of view since variable (high) linear polarization imprints a dominating source of contamination due to mode-conversion in the polarization analyzer. We present the results of calibrations of this contamination and limits on the circular polarization in several sources.