Depolarization and Faraday effects in AGN Jets

ABSTRACT Radio interferometric observations of active galactic nuclei (AGN) jets reveal the significant linear polarization of their synchrotron radiation that changes with frequency due to the Faraday rotation. It is generally assumed that such depolarization could be a powerful tool for studying the magnetized plasma in the vicinity of the jet. However, depolarization could also occur within the jet if the emitting and rotating plasma are cospatial (i.e. the internal Faraday rotation). Burn obtained very simple dependence of the polarization on the wavelength squared for the discrete source and resolved slab that is widely used for interpreting the depolarization of AGN jets. However, it ignores the influence of the non-uniform large-scale magnetic field of the jet on the depolarization. Under the simple assumptions about the possible jet magnetic field structures, we obtain the corresponding generalizations of Burn’s relation widely used for galaxies analysis. We show that the frequency dependences of the Faraday rotation measure and polarization angle in some cases allow to estimate the structures of the jets magnetic fields.