A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - I. Breaking the nπ-ambiguity

Abstract

The NRAO VLA Sky Survey (NVSS) rotation measure (RM) catalogue is invaluable for the study of cosmic magnetism. However, the RM values reported in it can be affected by nπ-ambiguity, resulting in deviations of the reported RM from the true values by multiples of ±652.9, rad m-2. We therefore set off to observationally constrain the fraction of sources in the RM catalogue affected by this ambiguity. New broad-band spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1-2, GHz, with 23 nπ-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to nπ-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarized and one source to be inconsistent with neither nπ-ambiguity nor reliable RM cases. By comparing the statistical distributions of the above two main classes, we devised a measure of how much a source's RM deviates from that of its neighbours: Δ/σ, which we found to be a good diagnostic of nπ-ambiguity. With this, we estimate that there are at least 50 sources affected by nπ-ambiguity among the 37 543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broad-band observations.