2 VLA observations

VLA (Thompson et al. 1980) observing dates and observational parameters are summarized in Table 1. The high resolution (A-array) Total Intensity images at 5 GHz for 0235-197 and 1203+043 were presented in Mantovani et al. (1992) while the lower resolution C-array, total intensity map for 0235-197 has been published in Morganti et al. (1993). However, because the polarization information was never presented in the earlier papers, we have summarized in the tables all the observational details and the derived parameters from those observations. Because of narrowband interference, the P (320MHz) band data were taken in spectral line mode. The data were edited to remove channels with interference. Bandpass corrections were determined from the calibrator source 3C 286 (1331+305) and applied to the spectral line database. A new "Channel 0'' database was then constructed and the data were calibrated for total intensity and polarization in the standard fashion (see, for example, Perley et al. 1989). Instrumental polarization calibration was done using the calibration sources 3C 48 (0134+329), 3C 138 (0521+166) and 3C 286 (1331+305) to get sufficient parallactic coverage across both days during which the target sources were observed. We assume that the instrument is stable between observing epochs.

Figure 1: VLA image of 0235-197 at 8.4 GHz. Contours are at -0.3, 0.3, 0.6, 1, 2, 4, 8, 16, 32, 64 mJybeam-1. The peak flux density is 45.4 mJybeam-1

The parallel hand (RR, LL) data were self-calibrated and imaged in the normal iterative manner. The complex gain corrections derived from self-calibration were also applied to the cross-hand (RL or LR) fringes. In turn, images in Stokes parameters I, Q, U and V were produced. Maps of the polarized flux density P=(Q²+U²)^1/2and position angle $\chi=0.5\times \tan^{-1}(U/Q)$, were then generated from the Q and U images. The Stokes V images were used to test the integrity of the calibration and self-calibration procedures and to diagnose problems due to interference.

At low frequencies, the primary beam of the antennas contains many background sources. In order to image the target sources of interest, it was necessary to image some of these background sources. The brightest (>20mJy at L band) background sources were identified from the NRAO VLA Sky Survey (NVSS) (Condon et al. 1998). This threshold is somewhat arbitrary but gives a reasonable number of secondary fields to image at 320MHz. We imaged a total of 26 fields for 0235-197 and 23 fields for 1203+043 in the AIPS mapping program, IMAGR -- one field containing the program source and the others on the brightest background sources. We were able to account for substantially all of the flux density seen on the shortest baselines in both cases and to obtain satisfactory convergence in the iterative self-calibration and imaging loop. The final images in all Stokes parameters (I, Q, U & V) of the target sources contained no obvious artefacts due to sidelobes from nearby confusing sources. The linear polarizations of the background sources have been checked in order to test for beam squint between R and L beams. The rms noises in the final images are within a factor of 3 of the expected thermal noises; this is entirely consistent with other observing programs at 320MHz. Together, these suggest that the images of the target sources are not greatly affected by confusion.

The low declination source 0235-197 was observed for only $\approx2$ hours at transit; consequently, some of the data were corrupted by cross-talk between antennas. These data, mostly on baselines with immediately adjacent antennas, were excised from the database during the iterative self-calibartion and imaging process.

The C (5 GHz), X (8.4 GHz) and U (15 GHz) band data were calibrated in the standard way using VLA calibrators and AIPS procedures. Polarimetric images were generated in a manner similar to that for the P band data.