GLEAM: The GaLactic and Extragalactic All-sky MWA survey

Abstract: GLEAM, the GaLactic and Extragalactic All-sky MWA survey, is a survey of the entire radio sky south of declination +25 deg at frequencies between 72 and 231 MHz, made with the Murchison Widefield Array (MWA) using a drift scan method that makes efficient use of the MWA's very large field-of-view. We present the observation details, imaging strategies and theoretical sensitivity for GLEAM. The survey ran for two years, the first year using 40 kHz frequency resolution and 0.5 s time resolution; the second year using 10 kHz frequency resolution and 2 s time resolution. The resulting image resolution and sensitivity depends on observing frequency, sky pointing and image weighting scheme. At 154 MHz the image resolution is approximately 2.5 x 2.2/cos(DEC+26.7) arcmin with sensitivity to structures up to ~10 deg in angular size. We provide tables to calculate the expected thermal noise for GLEAM mosaics depending on pointing and frequency and discuss limitations to achieving theoretical noise in Stokes I images. We discuss challenges, and their solutions, that arise for GLEAM including ionospheric effects on source positions and linearly polarised emission, and the instrumental polarisation effects inherent to the MWA's primary beam.

Overview of the GLEAM Survey

The GaLactic and Extragalactic All-sky MWA (GLEAM) survey, conducted using the Murchison Widefield Array (MWA), represents a substantial effort to map the radio sky south of a declination of +25° across a frequency range of 72 to 231 MHz. The GLEAM survey, utilizing a drift scan approach, capitalizes on the MWA's expansive field-of-view to efficiently collect data, which was recorded over two years with differing resolution setups.

Survey Design and Methodology

GLEAM's design involved dividing the sky into seven declination strips, with observational adjustments based on sky pointing and frequency. Observations in the first year were conducted with 40 kHz frequency resolution and 0.5-second time resolution, progressing to 10 kHz frequency resolution and 2-second time resolution in the second year. This approach allowed GLEAM to optimize sensitivity and resolution, which varies with frequency and the declination setting. Notably, the survey is capable of imaging structures up to approximately 10° in angular size.

A distinctive aspect of GLEAM is its use of the MWA's broad field-of-view and excellent snapshot u,v-coverage, positioning it as well-suited for extensive sky surveys. The observations were supplemented with periodic calibration on known bright sources to maintain the accuracy of the data collected.

Key Challenges and Solutions

GLEAM faced specific challenges inherent to low-frequency radio observations. These include ionospheric distortions affecting source positions, the impact of polarized emission, and inherent instrumental polarization effects. The survey implemented solutions such as incorporating zero-spacing information from existing surveys to address missing large-scale emission in deconvolution processes and using advanced imaging techniques to mitigate the primary beam's direction-dependent effects.

Sensitivity and Data Outputs

An analysis of the survey's theoretical sensitivity highlights GLEAM's capability to achieve an impressive thermal noise floor, conditioned by factors such as the observed region's systemic temperature and imaging weight schemes. The survey's extensive coverage suggests it will detect approximately 150,000 extragalactic sources, a noteworthy improvement in sensitivity over previous surveys at comparable frequencies.

GLEAM aims to produce comprehensive data products, including a Stokes I compact source catalogue, maps of diffuse extragalactic polarized and total intensity emissions, and extensive studies of the Galactic plane. Its wide frequency range and polarization analysis capabilities make it particularly valuable for various scientific inquiries involving AGNs, galaxy clusters, and Galactic phenomena.

Comparative Analysis and Future Implications

In the landscape of southern hemisphere radio surveys, GLEAM stands out for its unique combination of frequency coverage, sensitivity, and polarization capabilities. It surpasses previous surveys like the Molonglo Reference Catalogue in both depth and breadth. Complementing northern hemisphere surveys, GLEAM provides a comprehensive low-frequency map that is indispensable for the calibration and interpretation of forthcoming Square Kilometre Array (SKA) observations.

In summary, the GLEAM survey offers significant advancements in the study of the low-frequency sky, with implications for the exploration of both Galactic and extragalactic phenomena. It lays crucial groundwork for future astronomical explorations by furnishing a detailed sky model, enhancing our understanding in preparation for next-generation telescopes like the SKA.