Rapid polarization angle swings by relativistic aberration in extragalactic radio sources

Abstract

In the observational investigations of violently variable radio and optical objects (Blazars), the characteristics of polarized radiation can provide important information about the physical processes in the emitting regions. The rapid polarization angle swings observed in a number of these objects at radio (or optical) waveband are one kind of very special phenomena. In this paper the problem of rapid polarization angle swing of synchrotron radiation produced by relativistic shock, which propagates and “illuminates” the nonaxisymmetric (helical) magnetic field along the jet of compact radio sources, is discussed. It is shown that when β ps ∼ cosθ (β ps -velocity of the post-shock emitting region and θ—viewing angle), the maximum rate of polarization angle swing coincides not only with the minimum of polarization degree, but also with the minimum of flux density, i.e. the faster the polarization angle rotates, the lower both the polarization degree and the flux density. This model can explain the anticorrelation between polarization angle swing and polarization degree really observed. However, the variations in flux density usually observed are not anticorrelated with the polarization angle swing. In quite a few compact radio sources the observed polarization angle swings often occurred near the maxima of flux density or during its steadily declining phase. For these cases, alternative interpretations should be considered. For example, the two-component model consisting of the underlying jet and the relativistic shock within it may be more suitable to explain these swing phenomena.