Kavli Affiliate: George Efstathiou
| Summary:
We report the first upper limits on the power spectrum of 21-cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50-250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2$σ$ upper limits in eight spectral bands, spanning $5.6 leq z leq 24.4$ that are consistent with thermal noise at the $2σ$ level for $k gtrsim 0.6-0.9 hrm Mpc^-1$ (band dependent). Our tightest limit during Cosmic Dawn ($z>12$) is $1.13times 10^6 rm mK^2$ at ($k=0.55 hrm Mpc^-1, z=16.78$), and during the EoR ($5.5<z<12$) it is $1.78times 10^3 rm mK^2$ at ($k=0.70 hrm Mpc^-1, z=7.05$). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low-$k$ modes, resulting in the loss of modes from $k=0.35-0.55$ compared to Phase I data.
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