2006ApJ...641..710L

The Very Long Baseline Array (VLBA) polarimetric observations of extragalactic compact radio sources and quasars show rotation measure (RM) values that are much smaller than the enormous values predicted by the theory. This is under the assumption that the narrow-line region (NLR) and the broad-line region (BLR) are the Faraday medium in which the observed RM is produced. It is expected that the polarized emission in the parsec scale of these sources displays rest-frame RM values ranging from 10⁵ to 10⁸ rad/m2, whereas, the observed RMs of quasars and active galactic nuclei typically range from 10² to 10⁴ rad/m2 and, for most extragalactic sources, from 1 to 100 rad/m2. Recently, a dispersion model for highly relativistic jets has been proposed. The dependence of the observed radiation on the amount of relativistic motion, as well as on the geometry and intrinsic properties of the jets, has been analyzed. We employ several results of this dispersion model in order to discuss a possible solution to the RM problem. The RM and the electric vector of polarization angle (EVPA) in a relativistically moving plasma have been defined. We propose taking into account the relativistic motion of the medium in which the radiation is propagating, in order to avoid the large depolarization and also to permit the existence of high rest-frame rotation measure (RM') values.