Evidence of Ultra-faint Radio Frequency Interference in Deep 21~cm Epoch of Reionization Power Spectra with the Murchison Widefield Array

TL;DR

This paper reports deep upper limits on the 21cm power spectrum from the Epoch of Reionization using the Murchison Widefield Array, highlighting the detection of ultra-faint radio frequency interference and its implications for future observations.

Contribution

It introduces a detailed analysis of ultra-faint RFI contamination in 21cm power spectra and emphasizes the importance of modeling such interference for accurate EoR measurements.

Findings

Ultra-faint RFI signals can be identified in the power spectra.

Fainter RFI contamination is most evident in less than an hour of data.

Understanding RFI behavior is crucial for future 21cm cosmology experiments.

Abstract

We present deep upper limits from the 2014 Murchison Widefield Array (MWA) Phase I observing season, with a particular emphasis on identifying the spectral fingerprints of extremely faint radio frequency interference (RFI) contamination in the 21~cm power spectra (PS). After meticulous RFI excision involving a combination of the \textsc{SSINS} RFI flagger and a series of PS-based jackknife tests, our lowest upper limit on the Epoch of Reionization (EoR) 21~cm PS signal is $\Delta^2 \leq 1.61\cdot10^4 \text{ mK}^2$ at $k=0.258\text{ h Mpc}^{-1}$ at a redshift of 6.5 using 14.7 hours of data. By leveraging our understanding of how even fainter RFI is likely to contaminate the EoR PS, we are able to identify ultra-faint RFI signals in the cylindrical PS. Surprisingly this signature is most obvious in PS formed with less than an hour of data, but is potentially subdominant to other systematics in multiple-hour integrations. Since the total RFI budget in a PS detection is quite strict, this nontrivial integration behavior suggests a need to more realistically model coherently integrated ultra-faint RFI in PS measurements so that its potential contribution to a future detection can be diagnosed.