A Broadband Spectro-polarimetric View of the NVSS Rotation Measure
  Catalogue II: Effects of Off-axis Instrumental Polarisation
  The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue has enabled
numerous studies in cosmic magnetism, and will continue being a unique dataset
complementing future polarisation 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 broadband observations with the Karl G. Jansky Very
Large Array and the NVSS RM results for 23 sources. We found that two
unpolarised sources were reported as polarised at about 0.5% 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 polarisation in the NVSS
RM catalogue. By adopting the 0.5% above as the typical off-axis instrumental
polarisation 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 $\approx$ 10% to account for the off-axis instrumental
polarisation effect. This effect is more substantial for sources with lower
fractional polarisation, 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 polarised targets can be
reconciled. The remaining three are interpreted as showing hints of time
variabilities in RM.
