The Polarization of Variable Radio Sources at 8 GHz. I. The Observations

A program to monitor the linear polarization of variable radio sources at 8 GHz has been carried out at the University of Michigan since mid-1965. H ii regions and extended nonthermal sources were observed frequently in order to determine the instrumental polarization and to check the stability of the equipment. Five sources exhibited significant changes in polarization: 3C 120, 3C 273, 3C 279, 3C 345, and 3C 454.3; three sources, 3C 84, NRAO 150, and 3C 446 probably have variable polarizations. Changes in both the degree of polarization and the position angle of the polarization have been observed in these sources with time scales as short as 1 week. The most rapid variations in polarization usually occurred when the total flux density was increasing or had reached a maximum value. The amplitudes of the variations in polarized flux density were as great as 10 percent of the amplitudes of the variations in total flux density. Two sources, 3C 279 and 3C 454.3, exhibited large changes in polarized flux density during periods when the total flux densities were not changing significantly.