New EoR Power Spectrum Limits From MWA Phase II Using the Delay Spectrum Method and Novel Systematic Rejection

TL;DR

This paper presents new limits on the Epoch of Reionization 21 cm power spectrum using MWA Phase II data with the delay spectrum method, highlighting the importance of systematic mitigation for accurate cosmological constraints.

Contribution

First application of the delay spectrum approach to the same data set previously analyzed with imaging methods, revealing a critical systematic that affects power spectrum estimates.

Findings

Established new upper limits on the 21 cm power spectrum at specific k and z values.

Identified a previously unknown systematic that biases measurements negative.

Demonstrated the necessity of systematic mitigation for reliable cosmological inference.

Abstract

We present an analysis of Epoch of Reionization data from Phase II of the Murchison Widefield Array using the \texttt{simpleDS} delay spectrum pipeline. Prior work analyzed the same observations using the FHD/$\varepsilon$ppsilon imaging pipeline, and so the present analysis represents the first time that both principal types of 21 cm cosmology power spectrum estimation approaches have been applied to the same data set. Our limits on the 21 cm power spectrum amplitude span a range in $k$ space of $|k| < 1~h_{100}{\rm Mpc}^{-1}$ with a lowest measurement of $\Delta^2(k) \leq$ $4.58\times10^3$ mK$^2$ at $k = 0.190 h_{100}\rm{Mpc}^{-1}$ and $z = 7.14$. In order to achieve these limits, we need to mitigate a previously unidentified common mode systematic in the data set. If not accounted for, this systematic introduces an overall \emph{negative} bias that can make foreground contaminated measurements appear as stringent, noise-limited constraints on the 21 cm signal amplitude. The identification of this systematic highlights the risk in modeling systematics as positive-definite contributions to the power spectrum and in ``conservatively'' interpreting all measurements as upper limits.