Primary Beam Chromaticity in Hirax. I. Characterization From Simulations and Power Spectrum Implications

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an upcoming radio interferometric telescope designed to constrain dark energy through the 21 cm intensity mapping of baryon acoustic oscillations. Instrumental systematics must be controlled and carefully characterized to measure the 21 cm 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 side lobes and the main lobe. This paper investigates the chromaticity of the HIRAX Stokes I primary beam and its impact on the accurate measurement of the 21 cm power spectrum. To investigate the effect of chromaticity on the 21 cm power spectrum, we present a physically motivated beam modeling technique that utilizes a flexible basis derived from traditional optics, allowing for the accounting of higher-order radial and azimuthal structures in the primary beam. We investigate the impact of imperfect knowledge of the main lobe and side lobes' chromaticity in the power spectrum space by subtracting a simple foreground model in simulated snapshot visibilities to recover the H 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.