Biases in the polarization position angles in the NVSS point source catalogue

TL;DR

This study reveals a significant bias in polarization position angles in the NVSS point source catalogue, likely caused by CLEAN bias, affecting the interpretation of galactic magnetic field data from large-scale surveys.

Contribution

It identifies and analyzes the bias in polarization angles in NVSS data, attributing it to CLEAN bias and offsets, which impacts scientific analyses of galactic magnetic fields.

Findings

Significant bias toward angles multiple of 45 degrees in NVSS polarization data.

Bias is stronger in highly polarized sources.

CLEAN bias and offsets are probable causes of the observed bias.

Abstract

We have examined the statistics of the polarization position angles determined for point sources in the NRAO-VLA sky survey (NVSS) and find that there is a statistically significant bias toward angles which are multiples of 45 degrees. The formal probability that the polarization angles are drawn from a uniform distribution is exponentially small. When the sample of those NVSS sources with polarizations detected with a signal to noise $\geq$3 is split either around the median polarized flux density or the median fractional polarization, the effect appears to be stronger for the more highly polarized sources. Regions containing strong sources and regions at low galactic latitudes are not responsible for the non-uniform distribution of position angles. We identify CLEAN bias as the probable cause of the dominant effect, coupled with small multiplicative and additive offsets on each of the Stokes parameters. Our findings have implications for the extraction of science, such as information concerning galactic magnetic fields, from large scale polarization surveys.