The Tracking Tapered Gridded Estimator for the 21-cm power spectrum from the Murchison Widefield Array (MWA) drift scan observations – III. Improved upper limits at <i>z</i> = 8.2 from multiple pointings

Abstract

ABSTRACT We analyse zenith-pointing $(\delta =-26.7^{\circ })$ Murchison Widefield Array (MWA) $\nu _c=154.2 \, {\rm MHz}$ drift scan observations covering $349.0^{\circ } \le \alpha \le 70.0^{\circ }$ with 163 pointing centres (PCs) spaced by $0.5^{\circ }$. We measure $D_{\ell }$, the mean-squared angular brightness temperature fluctuations, as a function of $\alpha$. A broad peak at $\alpha \approx 50.0^{\circ }$ corresponds to the bright extended source Fornax A in the main lobe of the primary beam. A smaller peak at $\alpha \approx 5.0^{\circ }$ possibly corresponds to Fornax A in the first sidelobe. For $\alpha \le 22.0^{\circ }$ and $\ell \ge 200$, we find $D_{\ell } \propto \ell ^2$, which we interpret as Poisson fluctuations from point sources. We present $\Delta ^2(k)$, the mean-squared 21-cm brightness temperature fluctuations from the Epoch of Reionization, as a function of $\alpha$. Fornax A causes strong contamination near $\alpha \approx 50.5^{\circ }$, elsewhere several PCs are consistent with noise. The range $358.5^{\circ } \le \alpha \le 11.5^{\circ }$ is relatively foreground-free and best suited for EoR science. The PC at $\alpha = 11.0^{\circ }$ yields the best $2\sigma$ upper limit $\Delta ^{2}_{\rm UL}(k) = (173.13)^{2}\, {\rm mK^{2}}$ at $k = 0.161\, {\rm Mpc^{-1}}$. We incoherently combine 23 PCs to obtain $\Delta _{\rm UL}^2(k)=(98.67)^{2}\, {\rm mK}^{2}$ at $k=0.156\, {\rm Mpc}^{-1}$. This is the tightest upper limit from the MWA, being $\approx 3$ times lower than earlier MWA limits, but $\approx 2$ and $\approx 21$ times higher than the LOFAR and HERA limits, respectively, and $\approx 3$ orders of magnitude above theoretical predictions.