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 analyse zenith-pointing (δ = −26.7◦ ) Murchison Widefield Array (MWA) νc = 154.2 MHz drift scan observations covering 349.0◦ ≤ α ≤ 70.0◦ with 163 pointing centres (PCs) spaced by 0.5◦. We measure D , the mean-squared angular brightness temperature fluctuations, as a function of α. A broad peak at α ≈ 50.0◦ corresponds to the bright extended source Fornax A in the main lobe of the primary beam. A smaller peak at α ≈ 5.0◦ possibly corresponds to Fornax A in the first sidelobe. For α ≤ 22.0◦ and ≥ 200, we find D ∝ 2, which we interpret as Poisson fluctuations from point sources. We present 2(k), the mean-squared 21-cm brightness temperature fluctuations from the Epoch of Reionization, as a function of α. Fornax A causes strong contamination near α ≈ 50.5◦, elsewhere several PCs are consistent with noise. The range 358.5◦ ≤ α ≤ 11.5◦ is relatively foreground-free and best suited for EoR science. The PC at α = 11.0◦ yields the best 2σ upper limit 2 UL(k) = (173.13)2 mK2 at k = 0.161 Mpc−1. We incoherently combine 23 PCs to obtain 2 UL(k) = (98.67)2 mK2 at k = 0.156 Mpc−1. This is the tightest upper limit from the MWA, being ≈ 3 times lower than earlier MWA limits, but ≈ 2 and ≈ 21 times higher than the LOFAR and HERA limits, respectively, and ≈ 3 orders of magnitude above theoretical predictions