Message in a Bottle: Unveiling the Magneto-Ionic Complexity of AGNs
  through the Stokes QU-Fitting Technique
  Here, I overview one of the available techniques for the analysis of
broad-band spectropolarimetric data, the Stokes QU-fitting. Since broad-band
receivers have been installed at most radio facilities, the collection of radio
data, both the total intensity and the linear polarization, is revealing
interesting features in their spectra. The polarized light, and therefore its
properties, i.e. the fractional polarization and the polarized angle, are now
finally well sampled in wide wavelength ranges. The new complex behaviors
revealed by the data can be studied using the Stokes QU-fitting, which consists
of modeling the Stokes parameters Q and U using wavelength-dependent analytical
models, available in the literature. This technique provides a very good
diagnostic of the nature and structure of the magnetized plasma, with the
possibility to identify complex structures, internal or external, of the source
of study. A summary of the available and most used models describing the
polarization behavior, is presented. Moreover, some of the most significant
observational works which use this technique are also summarized.
