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Abstract

The Murchison Widefield Array (MWA) team has derived new upper limits on the spherically averaged power spectrum of the 21-cm signal at six redshifts in the range z ≈ 6.5–8.7. We use these upper limits and a Bayesian inference framework to derive constraints on the ionization and thermal state of the intergalactic medium (IGM) as well as on the strength of a possible additional radio background. We do not find any constraints on the state of the IGM for z ≳ 7.8 if no additional radio background is present. In the presence of such a radio background, the 95 per cent credible intervals of the disfavoured models at redshift ≳ 6.5 correspond to an IGM with a volume-averaged fraction of ionized regions below 0.6 and an average gas temperature ≲ 10³ K. In these models, the heated regions are characterized by a temperature larger than that of the radio background, and by a distribution with characteristic size ≲10h⁻¹ Mpc and a full width at half maximum (FWHM) of ≲30h⁻¹ Mpc. Within the same credible interval limits, we exclude an additional radio background of at least 0.008 per cent of the CMB at 1.42 GHz.