Primary Beam Chromaticity in HIRAX: I. Characterization from Simulations and Power Spectrum Implications

TL;DR

This paper characterizes the chromaticity of HIRAX's primary beam through simulations, demonstrating how modeling higher-order beam structures can mitigate foreground leakage in 21cm intensity mapping.

Contribution

It introduces a physically motivated beam modeling technique that accounts for complex beam structures, improving power spectrum measurements in 21cm cosmology.

Findings

Modeling up to octupolar azimuthal order reduces foreground leakage.

Simulated foreground subtraction recovers the HI power spectrum effectively.

Higher-order beam features are necessary for accurate power spectrum analysis.

Abstract

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an upcoming radio interferometric telescope designed to constrain dark energy through the 21cm intensity mapping of Baryon Acoustic Oscillations (BAO). Instrumental systematics must be controlled and carefully characterized to measure the 21cm power spectrum with fidelity and achieve high-precision constraints on the cosmological parameters. The chromaticity of the primary beam is one such complicated systematic, which can leak the power of spectrally smooth foregrounds beyond the ideal horizon limits due to the complex spatial and spectral structures of the sidelobes and the mainlobe. This paper studies the chromaticity of the HIRAX Stokes I primary beam and its effects on accurate measurements of the 21cm power spectrum. To investigate the effect of chromaticity in the 21cm power spectrum, we present a physically motivated beam modeling technique, which uses a flexible basis derived from traditional optics that can account for higher-order radial and azimuthal structures in the primary beam. We investigate the impact of imperfect knowledge of the mainlobe and sidelobes chromaticity in the power spectrum space by subtracting a simple foreground model in simulated snapshot visibilities to recover the H$\textsc{i}$ power spectrum. Additionally, we find that modeling up to the octupolar azimuthal order feature (fourth-order angular variation) in the primary beam is sufficient to reduce the leakage outside the wedge with minimal bias.