Polarized Brightness Distribution Over Cassiopeia a, the Crab Nebula, and Cygnus a at 1.55 CM Wavelength

Abstract

Observations with 1.7' resolution were made of Cassiopeia A, the Crab Nebula, and Cygnus A using the 140-foot reflector of the National Radio Astronomy Observatory and a polarimeter at 1.55-cm wave- length which measured directly the intensity with linear polarization oriented north-south and the Stokes parameters Q and U. These data were combined to determine the distribution of the total in- tensity of radiation, the intensity of linearly polarized radiation, and the degree and position angle of linear polarization for scans across the sources spaced at 0,8 intervals of declination. Cassiopeia A at 1.55-cm wavelength is a nearly circular source with enhanced regions of emission around the rim and remarkably symmetrical linear polarization of about 4.5 per cent around the rim and zero at the center of the source. The electric field is oriented circumferentially defining a radial mag- netic field associated with the polarized radiation. The total intensity contours of the Crab Nebula source are roughly elliptical and similar in shape and position to the outer envelope of the nebula, but the in- tensity of polarized radiation is concentrated in a smaller region centered about 0.5' south and 0.25' east of the center of the nebula with a maximum degree of polarization of about 16 per cent at position angle 154°. The distributions of total intensity and polarized intensity of Cygnus A are consistent with two point sources separated by 1.9' along position angle 109°. The total intensity of the east component is 12 per cent greater than that of the west component. The intensities of linearly polarized radiation from the two components are equal, but the polarization is nearly opposite