Understanding instrumental Stokes leakage in Murchison Widefield Array polarimetry

Abstract

This paper offers an electromagnetic, more specifically array theory, perspective on understanding strong instrumental polarization effects for planar low-frequency "aperture arrays" with the Murchison Widefield Array (MWA) as an example. A long-standing issue that has been seen here is significant instrumental Stokes leakage after calibration, particularly in Stokes Q at high frequencies. A simple model that accounts for interelement mutual coupling is presented which explains the prominence of Q leakage seen when the array is scanned away from zenith in the principal planes. On these planes, the model predicts current imbalance in the X (E-W) and Y (N-S) dipoles and hence the Q leakage. Although helpful in concept, we find that this model is inadequate to explain the full details of the observation data. This finding motivates further experimentation with more rigorous models that account for both mutual coupling and embedded element patterns. Two more rigorous models are discussed: the "full" and "average" embedded element patterns. The viability of the full model is demonstrated by simulating current MWA practice of using a Hertzian dipole model as a Jones matrix estimate. We find that these results replicate the observed Q leakage to approximately 2 to 5%. Finally, we offer more direct indication for the level of improvement expected from upgrading the Jones matrix estimate with more rigorous models. Using the average embedded pattern as an estimate for the full model, we find that Q leakage of a few percent is achievable.

Low-Frequency Observations of Linearly Polarized Structures in the Interstellar Medium Near the South Galactic Pole

“…The epoch 1 EoR data corresponds to a quiet period in the ionosphere whereas epoch 3 coincides with the arrival of a coronal mass ejection that propagated from the Sun (Kaplan et al 2015) and interacted with the ionosphere. For polarimetric studies, our interest is primarily in the 154 MHz data (low-band EoR data) becausethis band is less prone to polarization leakage than at higher frequencies, where inaccuracies in the MWA beam model become significant (Sutinjo et al 2015).…”

The First Murchison Widefield Array low-frequency radio observations of cluster scale non-thermal emission: the case of Abell 3667

“…Details of the technical design and specifications of the MWA are given by Tingay et al (2013) and a review of the performance properties and scientific goals is presented by Bowman et al (2013). Consisting of 128 'tiles' of 4 × 4 dipole antennas, which are concentrated in a dense core <1.5 km in diameter in order to achieve high surface brightness sensitivity.…”

“…335,149.135,179.855 and 225.935 MHz (hereafter 120,149,180 and 226 MHz). The MWA forms beams on the sky using analogue beam formers (Tingay et al 2013) whereby the beam is moved by changing the beam former delays by incremental amounts. This moves the primary beam centre and results in a change in the overall shape of the beam response.…”

“…This issue is amplified by the large delays used to generate beams at low declinations, the MFS nature of the data reduction process and the unknown number of non-functional dipoles in the array, which was not a metric that was recorded during the commissioning period. Further analysis is being carried out to determine the beam shape (Sutinjo, O'Sullivan & Lenc 2014) but at this time we must attempt to compensate for the unknown flux density errors introduced by the current primary beam model.…”

“…With the advent of instruments such as the Murchison Widefield Array (MWA; Bowman et al 2013;Tingay et al 2013) and the Low Frequency Array (van Haarlem et al 2013) we now have the ability to carry out such observations. In this study, we aim to observe a single galaxy cluster that is reported to host all three of these extended radio features, at low frequency using the MWA, as a preliminary step towards further such detections.…”