A 189 MHz, 2400 square degree polarization survey with the Murchison Widefield Array 32-element prototype

TL;DR

This paper presents a low-frequency polarization survey with the Murchison Widefield Array, revealing low polarization fractions in compact sources and extensive diffuse polarized emission, with implications for understanding local Galactic foregrounds.

Contribution

The study introduces a novel interferometric data analysis method for calibration, imaging, and deconvolution at 189 MHz, and provides the first polarization survey over 2400 square degrees at this frequency.

Findings

Detected polarization in one source at 1.8%

Most sources have polarization below 2% at 189 MHz

Diffuse polarized emission peaks at ~13 K with low rotation measures

Abstract

We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32-element prototype covering 2400 square degrees. The survey has a 15.6 arcmin angular resolution and achieves a noise level of 15 mJy/beam. We demonstrate a novel interferometric data analysis that involves calibration of drift scan data, integration through the co-addition of warped snapshot images and deconvolution of the point spread function through forward modeling. We present a point source catalogue down to a flux limit of 4 Jy. We detect polarization from only one of the sources, PMN J0351-2744, at a level of 1.8 \pm 0.4%, whereas the remaining sources have a polarization fraction below 2%. Compared to a reported average value of 7% at 1.4 GHz, the polarization fraction of compact sources significantly decreases at low frequencies. We find a wealth of diffuse polarized emission across a large area of the survey with a maximum peak of ~13 K, primarily with positive rotation measure values smaller than +10 rad/m^2. The small values observed indicate that the emission is likely to have a local origin (closer than a few hundred parsecs). There is a large sky area at 2^h30^m where the diffuse polarized emission rms is fainter than 1 K. Within this area of low Galactic polarization we characterize the foreground properties in a cold sky patch at $(\alpha,\delta) = (4^h,-27^\circ.6)$ in terms of three dimensional power spectra