The Polarization of Variable Radio Sources at 8 GHz. II. Interpretation

The variations in polarization of extragalactic radio sources observed at 8 GHz are compared with the of a model of the expanding-source type and found to have a number of characteristics in common. The rapid variations in polarization are interpreted as being produced primarily by the decrease in radio optical thickness of the source with time. The amplitudes of the changes in polarization indicate that the magnetic fields in these objects must be relatively homogeneous over large regions. The apparent low degrees of polarization generally derived for the variable components of these sources are believed to be due to the depolarization effect of synchrotron sell-absorption. The absence of appreciable Faraday depolarization and the fact that the Razin-Tsytovich effect is not observed in the variable components provides an upper limit on the density of thermal electrons present in these variable components. This upper limit is several orders of magnitude less than the density estimates obtained from optical emission-line data. The derived upper limit to the amount of nonrelativistic material ejected in each outburst is of the order of 0.5 1i'o In the case of 3C 279, data on polarization position angle over a wide range of wavelengths allow an upper limit of 10 to be placed on the density of thermal electrons surrounding the variable component. The large disparity between radio and optical estimates of the density of ionized gas appears to require either that this gas be confined to "blobs" or filaments, or that it form a disklike distribution which does not completely envelop the central, variable component of the source.