The Tracking Tapered Gridded Estimator for the 21-cm power spectrum from MWA drift scan observations I: Validation and preliminary results

Drift scan observations provide the broad sky coverage and instrumental stability needed to measure the Epoch of Reionization (EoR) 21-cm signal. In such observations, the telescope's pointing center (PC) moves continuously on the sky. The Tracking Tapered Gridded Estimator (TTGE) combines observations from different PC to estimate P(k⊥, k∥) the 21-cm power spectrum, centered on a tracking center (TC) which remains fixed on the sky. The tapering further restricts the sky response to a small angular region around TC, thereby mitigating wide-field foregrounds. Here we consider 154.2 \, MHz ($z = 8.2$) Murchison Widefield Array (MWA) drift scan observations. The periodic pattern of flagged channels, present in MWA data, is known to introduce artefacts which pose a challenge for estimating P(k⊥, k∥). We demonstrate that the TTGE is able to recover P(k⊥, k∥) without any artefacts, and estimate $P(k)$ within $5 %$ accuracy over a large k-range. We also present preliminary results for a single PC, combining 9 nights of observation (17 \, min total). We find that P(k⊥, k∥) exhibits streaks at a fixed interval of k∥=0.29 \, Mpc⁻¹, which matches Δ νₚₑᵣ=1.28 \, MHz that is the period of the flagged channels. The streaks are not as pronounced at larger k∥, and in some cases they do not appear to extend across the entire k⊥ range. The rectangular region 0.05 ≤ k⊥ ≤ 0.16 \, Mpc⁻¹ and 0.9 ≤ k∥ ≤ 4.6 \, Mpc⁻¹ is found to be relatively free of foreground contamination and artefacts, and we have used this to place the 2σ upper limit Δ²(k) < (1.85 × 10⁴)²\, mK² on the EoR 21-cm mean squared brightness temperature fluctuations at k=1 \, Mpc⁻¹.