Measuring the global 21-cm signal with the MWA-I: improved measurements of the Galactic synchrotron background using lunar occultation

TL;DR

This study uses lunar occultation observed by the MWA to improve measurements of the Galactic synchrotron background, advancing techniques for detecting the global 21-cm signal from the Epoch of Reionisation.

Contribution

It introduces a refined lunar occultation method with better modeling of earthshine and image differencing, enhancing the measurement of sky brightness temperature.

Findings

Measured lunar brightness temperature as 180 ± 12 K.

Determined Galactic synchrotron spectral index as -2.64 ± 0.14.

Verified the lunar occultation technique's potential for EoR detection.

Abstract

We present early results from a project to measure the sky-averaged (global), redshifted $21\,$cm signal from the Epoch of Reionisation (EoR), using the Murchison Widefield Array (MWA) telescope. Because interferometers are not sensitive to a spatially-invariant global average, they cannot be used to detect this signal using standard techniques. However, lunar occultation of the radio sky imprints a spatial structure on the global signal, allowing us to measure the average brightness temperature of the patch of sky immediately surrounding the Moon. In this paper we present one night of Moon observations with the MWA between 72 - 230 MHz and verify our techniques to extract the background sky temperature from measurements of the Moon's flux density. We improve upon previous work using the lunar occultation technique by using a more sophisticated model for reflected `earthshine' and by employing image differencing to remove imaging artefacts. We leave the Moon's (constant) radio brightness temperature as a free parameter in our fit to the data and as a result, measure $T_{\rm{moon}} = 180 \pm 12 $ K and a Galactic synchrotron spectral index of $-2.64\pm0.14$, at the position of the Moon. Finally, we evaluate the prospects of the lunar occultation technique for a global EoR detection and map out a way forward for future work with the MWA.