Study of HI and nuisance outliers in nonstop observations for MWA EoR (SHINONOME). I. First analysis and field status at the cosmic dawn frequency

We analyze drift-scan data observed by the Murchison Widefield Array (MWA) in its lowest frequency band, spanning 73.6–104.3 MHz. The data were obtained to constrain the physical state of neutral hydrogen in the intergalactic medium during the Cosmic Dawn. Using zenith-pointing drift-scan observations covering right ascensions from -45^∘ to +75^∘ and a declination of -26∘.8, we characterize the low-frequency radio sky to identify optimal fields for 21 cm signal analysis. The data shows that large-scale diffuse emission dominates the short baseline visibility RA < 10^∘. At RA > 20^∘, visibilities are dominated by Fornax A. Fornax A serves as a powerful calibrator source, and errors in the gain solutions are minimized around RA = 50^∘, where the highest dynamic range is achieved, as the power spectrum is affected by the gain errors. Using a simple model, we estimate that the gain solution accuracy must be better than 0.01% to enable detection of the standard 21 cm power spectrum from the Cosmic Dawn. We propose the sky direction around (RA, Dec) = (10^∘–40^∘, -26∘.8) as candidate regions for MWA ultralow observations. Furthermore, we show that some baselines exhibit unexpected frequency structure at the ultralow band, and the delay power spectrum of individual baselines provides an effective diagnostic for identifying those outliers.