3 Observations and data reduction

We used the VLA in its A (27 km) configuration on 1998 March 23 and 24 to make the observations. All sources were observed at two frequencies in the 5 GHz band of the VLA (4535 and 4885 MHz) and at two frequencies in the 8 GHz band (8085 and 8335 MHz). Bandwidths were 50 MHz for all frequencies. Each source was observed for 15 minutes at 8 GHz and 8 minutes at 5 GHz.

An important limitation of using the VLA in the A configuration is the short spacing limit. This implies a maximum size on which we have information of about 10'' at 5 GHz and 6'' at 8 GHz. Although this is important for the large angular scale sources, it will have a negligible effect on bright, small components such as the hot spots and cores. The data were gain-calibrated using 3C 286. We used multiple scans of the calibrator 1745+173 to determine the on-axis antenna polarization response. Two scans of 3C 286 separated in time by 7 hours were used to measure the absolute linear polarization position angles.

We used the Astronomical Image Processing System (AIPS) to process the data. After calibration the data were edited and self-calibrated using standard procedures to improve image dynamic range. The first few self-calibration iterations involved phase self-calibration using a model derived from the same data. Natural weighting of the gridded visibilities was employed. The AIPS task IMAGR, in which the CLEAN algorithm is implemented, was used to deconvolve the images. The FWHM of the Gaussian restoring beams are shown in the bottom-left corners of Figs. 6-32. We synthesized images of the three Stokes parameters, I, Q and U, and all images were CLEANed down to the noise-levels. The achieved noise is 25 $\mu$Jy/beam at 8 GHz and 50 $\mu$Jy/beam at 5 GHz. The resolution of the observations is 0.23'' for the 8.2 GHz maps, and 0.43'' for the 4.7 GHz maps.

Total intensity maps were created using the combined data from the two frequencies per band and the mean frequencies in each band. In order to make spectral index maps we convolved the 8 GHz image with the Gaussian restoring beam of the 5 GHz image. Since the spectrum of the sources can be approximated by a power law, the spectral index $\alpha$ is defined as as $I_{\nu} \propto{\nu}^{\alpha}$, where $I_{\nu}$ is the surface brightness at frequency $\nu$. We calculated two-point spectral index values only for pixels with surface brightness exceeding 3.5$\sigma$ (where $\sigma$ is the measured off-source rms on an image) at both frequencies.

We used position angles for the polarized intensity for three frequencies (4535, 4885 and 8200 MHz) to derive rotation measures. The rotation measures were derived using the AIPS task RM. For the hot-spots, rotation measures were derived for the position of peak intensity.