Simulations of primary beam effects on the cosmic bispectrum phase observed with the Hydrogen Epoch of Reionization Array

TL;DR

This study uses simulations to analyze how antenna primary beam effects, especially mutual coupling, influence the cosmic bispectrum phase measurements in 21cm Epoch of Reionisation observations, revealing significant foreground leakage.

Contribution

It demonstrates the impact of mutual coupling in antenna beams on foreground leakage into the EoR window, highlighting the importance of beam effects in calibration-free approaches.

Findings

Mutual coupling causes foreground power leakage up to 4 orders of magnitude.

Leakage is confined to low k modes (<0.3 h Mpc^{-1}) for certain baseline triads.

Beam-to-beam variation is not the main source of leakage in the EoR window.

Abstract

The 21~cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation. The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. Closure quantities may circumvent the calibration requirements but may be, however, affected by direction dependent effects, particularly antenna primary beam responses. This work investigates the impact of antenna primary beams affected by mutual coupling on the closure phase and its power spectrum. Our simulations show that primary beams affected by mutual coupling lead to a leakage of foreground power into the EoR window, which can be up to $\sim4$ orders magnitude higher than the case where no mutual coupling is considered. This leakage is, however, essentially confined at $k < 0.3$~$h$~Mpc$^{-1}$ for triads that include 29~m baselines. The leakage magnitude is more pronounced when bright foregrounds appear in the antenna sidelobes, as expected. Finally, we find that triads that include mutual coupling beams different from each other have power spectra similar to triads that include the same type of mutual coupling beam, indicating that beam-to-beam variation within triads (or visibility pairs) is not the major source of foreground leakage in the EoR window.