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 = 154.2MHz drift scan observations covering 349.0◦ ≤ α ≤ 70. 0◦ with 163 pointing centres (PCs) spaced by 0. 5◦. We measure Dℓ, 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 ℓ ≥ 200, we find Dℓ ∝ ℓ2, 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◦2 yields the best 2 σ upper limit 2 UL (k) = (173. 13)2 mK2 at k = 0. 161Mpc−1. We incoherently combine 23 PCs to obtain UL (k) = (98. 67)2 mK 2 at k = 0. 156Mpc −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.