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 $z=8.2$ from multiple pointings

Abstract

We analyze zenith-pointing $(\delta =-26.7^{\circ })$ Murchison Widefield Array (MWA) ${\nu }_c=154.2\mathrm{MHz}$ drift scan observations covering $349.0^{\circ }\le \alpha \le 70.0^{\circ }$ with 163 pointing centers (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 ${\Delta }_{\mathrm{UL}}^2(k)=(173.13{)}^2\mathrm{m}{\mathrm{K}}^2$ at $k=0.161\mathrm{Mp}{\mathrm{c}}^{-1}$. We incoherently combine $23$ PCs to obtain ${\Delta }_{\mathrm{UL}}^2(k)=(98.67{)}^2{\mathrm{mK}}^2$ at $k=0.156{\mathrm{Mpc}}^{-1}$. This is the tightest upper limit from the MWA, being $\approx 3$ times lower than earlier MWA limits at $z=8.2$, but $\approx 2$ and $\approx 21$ times higher than the LOFAR and HERA limits, respectively, and $\approx 3$ orders of magnitude above theoretical predictions.

Accepted in MNRAS. Supplementary materials are available online