From the NVSS RM Catalogue to Future Polarisation Surveys

TL;DR

This paper evaluates the robustness of the Taylor RM catalogue using new broadband VLA observations, highlighting the importance of calibrating off-axis leakage for future high-fidelity polarisation surveys like SKA.

Contribution

It provides an observational assessment of the Taylor RM catalogue's systematic effects and offers calibration insights for upcoming large-scale polarisation surveys.

Findings

Off-axis leakage significantly affects RM measurements.

Proper calibration is essential for low fractional polarisation sources.

Broadband observations improve RM robustness.

Abstract

With rotation measure (RM) towards 37,543 polarised sources, the Taylor et al. (2009) RM catalogue has been widely exploited in studies of the foreground magneto-ionic media. However, due to limitations imposed by observations in survey mode in the narrowband era, the listed RM values are inevitably affected by various systematic effects. With new Karl G. Jansky Very Large Array (VLA) broadband spectro-polarimetric observations at L-band, we set off to observationally examine the robustness of the Taylor catalogue. This would facilitate combinations and comparisons of it with results from current and future polarisation surveys such as Polarization Sky Survey of the Universe's Magnetism (POSSUM), VLA Sky Survey (VLASS), and the eventual Square Kilometre Array (SKA). Our on-axis pointed observations, in conjunction with simulations, allowed us to estimate the impact of off-axis polarisation leakage on the measured RM values. This demonstrates the importance to properly calibrate for the off-axis leakage terms in future all-sky polarisation surveys, in order to obtain high fidelity polarisation information from sources down to low fractional polarisation.