SIMBAD references

Missing frequency channels pose a problem in estimating the redshifted 21-cm power spectrum $P(k_\perp ,k_\parallel)$ from radio-interferometric visibility data. This is particularly severe for the Murchison Widefield Array (MWA), which has a periodic pattern of missing channels that introduces spikes along $k_\parallel$. The Tracking Tapered Gridded Estimator (TTGE) overcomes this by first correlating the visibilities in the frequency domain to estimate the multifrequency angular power spectrum (MAPS) $C_\ell (\Delta \nu)$ that has no missing frequency separation $\Delta \nu$. We perform a Fourier transform along $\Delta \nu$ to estimate $P(k_\perp ,k_\parallel)$. Simulations demonstrate that the TTGE can estimate $P(k_\perp ,k_\parallel)$ without any artefacts due to missing channels. However, the spikes persist for the actual foreground-dominated data. A detailed investigation, considering both simulations and actual data, reveals that the spikes originate from a combination of the missing channels and the strong spectral dependence of the foregrounds. We propose and demonstrate a technique to mitigate the spikes. Applying this, we find the values of $P(k_\perp ,k_\parallel)$ in the region $0.004 \le k_\perp \le 0.048\, {\rm Mpc^{-1}}$ and $k_\parallel > 0.35 \, {\rm Mpc^{-1}}$ to be consistent with zero within the expected statistical fluctuations. We obtain the $2\sigma$ upper limit of $\Delta _{\rm UL}^2(k)=(934.60)^2\, {\rm mK^2}$ at $k=0.418\, {\rm Mpc^{-1}}$ for the mean-squared brightness temperature fluctuations of the $z=8.2$ epoch of reionization (EoR) 21-cm signal. This upper limit is from $\sim 17$ min of observation for a single pointing direction. We expect tighter constraints when we combine all 162 different pointing directions of the drift scan observation.