Spectral polarization properties of optically thin synchrotron radiation

Abstract

The spectral polarization of optically thin synchrotron emission from an ensemble of relativistic electron gyrating in a uniform magnetic field was first treated by Westfold (1959). Several highly polarized extragalactic objects, called 'blazars' by Angel and Stockman (1980), show curvature in their nonthermal continua. Although the emission mechanism is believed to be synchrotron radiation, the origin of the curvature is still uncertain. If it can be established that in some objects the curvature is intrinsic to the synchrotron radiation, important information about the magnetic field distribution in these sources can be obtained by carefully measuring both spectral index and polarization as functions of frequency. Results obtained by Nordsieck (1976) regarding the polarization have been used by Rieke et al. (1977) to argue against a synchrotron origin for the optical continuum of PKS 0735+178. The present investigation shows how a synchrotron origin can be reconciled with the polarization behavior.