Multi-frequency Polarization Studies of AGN Jets

Throughout most of the history of radio interferometry studies of Active Galactic Nuclei (AGN) using VLBI, attempts to extract physically meaningful information from VLBI observations have been dominated by visibility-based approaches to analysis, such as model fitting of the self-calibrated visibility data. This approach has been extremely valuable in multi-epoch studies of superluminal motion, for example; however, as polarization-sensitive and multi-frequency studies of the parsec-scale jets of AGN have become more common, it has often proven difficult to unambiguously relate features observed in total intensity and polarization, or at multiple frequencies. This has necessitated the development of new image-based analysis techniques, as well as studies aimed at improving our understanding of the uncertainties in quantities derived from images. These new techniques have provided a wide range of key information about the relativisitic jets of AGN on parsec scales, including measurements of typical magnetic-field strengths in the VLBI core components and information about the three-dimensional magnetic-field structures of the jets. These properties of the jets are, in turn, intrinsically related to fundamental questions such as the formation, launching and collimation of the jet structures