# A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue I: Breaking the n{\pi}-ambiguity

## TL;DR

This study uses broadband spectro-polarimetric observations to identify and quantify nπ-ambiguity in the NVSS RM catalogue, improving the accuracy of cosmic magnetic field measurements.

## Contribution

It introduces a new diagnostic measure for nπ-ambiguity and provides the first observational constraints on its prevalence in the NVSS RM catalogue.

## Key findings

- Identified 9 sources with erroneous RM due to nπ-ambiguity
- Estimated at least 50 sources affected by nπ-ambiguity in the catalogue
- Developed a diagnostic measure, Δ/σ, for nπ-ambiguity detection

## 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 n$\pi$-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 broadband spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, with 23 n$\pi$-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to n$\pi$-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarised and one source to be inconsistent with neither n$\pi$-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: $\Delta/\sigma$, which we found to be a good diagnostic of n$\pi$-ambiguity. With this, we estimate that there are at least 50 sources affected by n$\pi$-ambiguity among the 37,543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broadband observations.