2021MNRAS.501.3378P

We apply the Tapered Gridded Estimator for estimating the cosmological 21-cm power spectrum from 150MHz GMRT observations, which corresponds to the neutral hydrogen (H I) at redshift z = 8.28. Here, TGE is used to measure the multifrequency angular power spectrum (MAPS) C_l(Δν) first, from which we estimate the 21-cm power spectrum P(k_⊥, k_||). The data here are much too small for a detection, and the aim is to demonstrate the capabilities of the estimator. We find that the estimated power spectrum is consistent with the expected foreground and noise behaviour. This demonstrates that this estimator correctly estimates the noise bias and subtracts this out to yield an unbiased estimate of the power spectrum. More than 47 per cent of the frequency channels had to be discarded from the data owing to radio-frequency interference, however, the estimated power spectrum does not show any artefacts due to missing channels. Finally, we show that it is possible to suppress the foreground contribution by tapering the sky response at large angular separations from the phase centre. We combine the k modes within a rectangular region in the 'EoR window' to obtain the spherically binned averaged dimensionless power spectra Δ²(k) along with the statistical error σ associated with the measured Δ²(k). The lowest k-bin yields Δ²(k)=(61.47)² K² at k=1.59 Mpc^–1, with σ =(27.40)² K². We obtain a 2 σ upper limit of (72.66)² K² on the mean squared H I 21-cm brightness temperature fluctuations at k=1.59 Mpc^–1.