Search for the Epoch of Reionisation with HERA: Upper Limits on the Closure Phase Delay Power Spectrum

TL;DR

This paper reports upper limits on the Epoch of Reionisation 21 cm power spectrum using closure phase techniques with HERA data, offering a calibration-independent approach to detect the faint signal.

Contribution

It introduces a closure phase-based delay spectrum method for EoR detection, providing an independent and simpler calibration approach compared to standard visibility-based analyses.

Findings

Placed approximate 95% confidence upper limits on the 21 cm power spectrum at z=7.7.

Demonstrated the feasibility of detecting the EoR with the full HERA array using closure phases.

Showed that closure phase analysis can serve as a calibration-independent validation method.

Abstract

Radio interferometers aiming to measure the power spectrum of the redshifted 21 cm line during the Epoch of Reionisation (EoR) need to achieve an unprecedented dynamic range to separate the weak signal from overwhelming foreground emissions. Calibration inaccuracies can compromise the sensitivity of these measurements to the effect that a detection of the EoR is precluded. An alternative to standard analysis techniques makes use of the closure phase, which allows one to bypass antenna-based direction-independent calibration. Similarly to standard approaches, we use a delay spectrum technique to search for the EoR signal. Using 94 nights of data observed with Phase I of the Hydrogen Epoch of Reionization Array (HERA), we place approximate constraints on the 21 cm power spectrum at $z=7.7$. We find at 95% confidence that the 21 cm EoR brightness temperature is $\le$(372)$^2$ "pseudo" mK$^2$ at 1.14 "pseudo" $h$ Mpc$^{-1}$, where the "pseudo" emphasises that these limits are to be interpreted as approximations to the actual distance scales and brightness temperatures. Using a fiducial EoR model, we demonstrate the feasibility of detecting the EoR with the full array. Compared to standard methods, the closure phase processing is relatively simple, thereby providing an important independent check on results derived using visibility intensities, or related.