[Paper Review] The MWA Long Baseline Epoch of Reionisation Survey: I. Improved Source Catalogue for the EoR 0 field

C. Lynch, Timothy J. Galvin+7 more|arXiv (Cornell University)|Oct 15, 2021

Radio Astronomy Observations and TechnologyPhysics and Astronomy124 references21 citations

TL;DR

This paper presents the MWA Long Baseline Epoch of Reionisation Survey (LoBES), which delivers a high-resolution, multi-frequency source catalogue for the EoR0 field using the MWA Phase II array. By improving spectral and structural modelling of extended and bright foreground sources—especially those in the primary beam sidelobes—it reduces residual foreground power in MWA Phase I data, particularly on small angular scales, thereby enhancing calibration accuracy for Epoch of Reionisation experiments.

ABSTRACT

One of the principal systematic constraints on the Epoch of Reionisation (EoR) experiment is the accuracy of the foreground calibration model. Recent results have shown that highly accurate models of extended foreground sources, and including models for sources in both the primary beam and its sidelobes, are necessary for reducing foreground power. To improve the accuracy of the source models for the EoR fields observed by the Murchison Widefield Array (MWA), we conducted the MWA Long Baseline Epoch of Reionisation Survey (LoBES). This survey consists of multi-frequency observations of the main MWA EoR fields and their eight neighbouring fields using the MWA Phase II extended array. We present the results of the first half of this survey centred on the MWA EoR0 observing field (centred at RA(J2000) 0 h, Dec(J2000) -27 deg). This half of the survey covers an area of 3069 degrees$^2$, with an average rms of 2.1 mJy beam$^{-1}$. The resulting catalogue contains a total of 80824 sources, with 16 separate spectral measurements between 100 and 230 MHz, and spectral modelling for 78$\%$ of these sources. Over this region we estimate that the catalogue is 90$\%$ complete at 32 mJy, and 70$\%$ complete at 10.5~mJy. The overall normalised source counts are found to be in good agreement with previous low-frequency surveys at similar sensitivities. Testing the performance of the new source models we measure lower residual rms values for peeled sources, particularly for extended sources, in a set of MWA Phase I data. The 2-dimensional power spectrum of these data residuals also show improvement on small angular scales -- consistent with the better angular resolution of the LoBES catalogue. It is clear that the LoBES sky models improve upon the current sky model used by the Australian MWA EoR group for the EoR0 field.

Motivation & Objective

• . The primary objective is to improve foreground calibration models for the Epoch of Reionisation (EoR) by creating a high-resolution, multi-frequency source catalogue for the MWA EoR0 field.
• The study addresses the critical limitation of current sky models, which often misrepresent extended and bright sources, especially those in the primary beam sidelobes.
• It aims to reduce foreground contamination in 21 cm power spectrum measurements by providing accurate spectral and structural models for sources across 100–230 MHz.
• The research seeks to enhance the fidelity of source peeling and foreground subtraction in EoR experiments, particularly for the Murchison Widefield Array (MWA).
• It targets a 90% completeness level at 32 mJy and 70% at 10.5 mJy, with spectral modelling for 78% of sources, to support future EoR detection efforts.

Proposed method

• . The LoBES survey uses the MWA Phase II extended array to conduct multi-frequency observations across 100–230 MHz, covering 3069 deg² around the EoR0 field.
• A total of 80,824 sources were detected, with 16 spectral measurements per source, enabling detailed spectral index and curvature modelling.
• Sources were cross-matched and modelled using multi-component techniques, including Gaussian and shapelet-based models for extended sources.
• The survey employs a wideband imaging pipeline to produce deep 60 MHz images with an average rms of 2.1 mJy beam⁻¹.
• The new source catalogue was validated by comparing residual power spectra from MWA Phase I data using both the current Australian MWA EoR sky model and the LoBES model.
• The performance was assessed via 2D power spectrum analysis, focusing on residual foreground power within the foreground wedge, particularly on small angular scales.

Experimental results

Research questions

• RQ1 . To what extent does the LoBES source catalogue improve foreground calibration accuracy compared to the current Australian MWA EoR sky model?
• RQ2 How effective are the multi-frequency, multi-component source models in reducing residual foreground power in MWA Phase I data?
• RQ3 What is the completeness and spectral fidelity of the LoBES catalogue at different flux density thresholds?
• RQ4 How do the improved models affect the 2D power spectrum of residuals, especially on small angular scales?
• RQ5 What role do extended and sidelobe sources play in residual foreground contamination, and how well are they modelled in the new catalogue?

Key findings

• . The LoBES catalogue achieves 90% completeness at 32 mJy and 70% completeness at 10.5 mJy, with a median sensitivity of 2.1 mJy beam⁻¹.
• Spectral modelling was successfully applied to 78% of the 80,824 sources, with spectral indices consistent with previous literature for sources best fitted by a power-law.
• The 2D power spectrum of residual foregrounds shows a significant reduction in power on small angular scales when using the LoBES model compared to the current sky model.
• Residual rms values are lower for peeled sources, especially for extended sources, indicating improved calibration fidelity.
• The improvement is most pronounced within the foreground wedge, confirming that better source structure and spectral models reduce foreground leakage.
• The study identifies diffuse Galactic emission and beam model inaccuracies as potential remaining systematics limiting residual power on large angular scales.