Calibration and 21-cm Power Spectrum Estimation in the Presence of Antenna Beam Variations

TL;DR

This paper investigates how antenna beam variations, especially broken dipoles in the MWA, impact 21-cm power spectrum measurements for detecting the Epoch of Reionisation, highlighting calibration errors as a significant challenge.

Contribution

It introduces a framework to estimate the impact of beam modeling residuals on 21-cm power spectrum estimation, specifically analyzing broken dipoles in the MWA.

Findings

Incorrect beam modeling causes a bias of ~10^3 mK^2 in the 2D power spectrum.

Residuals from broken dipoles are significantly above the expected EoR signal level.

Calibration accuracy is crucial for reliable EoR detection.

Abstract

Detecting a signal from the Epoch of Reionisation (EoR) requires an exquisite understanding of galactic and extra-galactic foregrounds, low frequency radio instruments, instrumental calibration, and data analysis pipelines. In this work we build upon existing work that aims to understand the impact of calibration errors on 21-cm power spectrum (PS) measurements. It is well established that calibration errors have the potential to inhibit EoR detections by introducing additional spectral features that mimic the structure of EoR signals. We present a straightforward way to estimate the impact of a wide variety of modelling residuals in EoR PS estimation. We apply this framework to the specific case of broken dipoles in Murchison Widefield Array (MWA) to understand its effect and estimate its impact on PS estimation. Combining an estimate of the percentage of MWA tiles that have at least one broken dipole (15%-40%) with an analytic description of beam errors induced by such dipoles, we compute the residuals of the foregrounds after calibration and source subtraction. We find that that incorrect beam modelling introduces bias in the 2D-PS on the order of $\sim 10^3\, \mathrm{mK}^2 \,h^{-3}\, \mathrm{Mpc}^{3}$. Although this is three orders of magnitude lower than current lowest limits, it is two orders of magnitude higher than the expected signal. Determining the accuracy of both current beam models and direction dependent calibration pipelines is therefore crucial in our search for an EoR signal.