2021MNRAS.505.4775Y

Observations in the lowest Murchison Widefield Array (MWA) band between 75 and 100 MHz have the potential to constrain the distribution of neutral hydrogen in the intergalactic medium at redshift ∼13-17. Using 15 h of MWA data, we analyse systematics in this band such as radio-frequency interference (RFI), ionospheric and wide field effects. By updating the position of point sources, we mitigate the direction-independent calibration error due to ionospheric offsets. Our calibration strategy is optimized for the lowest frequency bands by reducing the number of direction-dependent calibrators and taking into account radio sources within a wider field of view. We remove data polluted by systematics based on the RFI occupancy and ionospheric conditions, finally selecting 5.5 h of the cleanest data. Using these data, we obtain 2σ upper limits on the 21 cm power spectrum in the range of 0.1 h Mpc^–1 \lessapprox k \lessapprox 1 h Mpc^–1 and at z = 14.2, 15.2, and 16.5, with the lowest limit being 6.3 ×10⁶mK² at k=0.14 h Mpc^–1 and at z = 15.2 with a possibility of a few per cent of signal loss due to direction-independent calibration.