A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue-II. Effects of off-axis instrumental polarization

The NRAO VLA Sky Survey (NVSS) rotation measure (RM) catalogue has enabled numerous studies in cosmic magnetism, and will continue being a unique data set complementing future polarization surveys. Robust comparisons with these new surveys will however require further understandings in the systematic effects present in the NVSS RM catalogue. In this paper, we make careful comparisons between our new on-axis broad-band observations with the Karl G. Jansky Very Large Array and the NVSS RM results for 23 sources. We found that two unpolarized sources were reported as polarized at about 0.5 per cent level in the RM catalogue, and noted significant differences between our newly derived RM values and the catalogue values for the remaining 21 sources. These discrepancies are attributed to off-axis instrumental polarization in the NVSS RM catalogue. By adopting the 0.5 per cent above as the typical off-axis instrumental polarization amplitude, we quantified its effect on the reported RMs with a simulation, and found that on average the RM uncertainties in the catalogue have to be increased by 10% to account for the off-axis instrumental polarization effect. This effect is more substantial for sources with lower fractional polarization, and is a function of the source's true RM. Moreover, the distribution of the resulting RM uncertainty is highly non-Gaussian. With the extra RM uncertainty incorporated, we found that the RM values from the two observations for most (18 out of 21) of our polarized targets can be reconciled. The remaining three are interpreted as showing hints of time variabilities in RM.