Ajith Sampath

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.

The Hydrogen Intensity and Real-time Analysis Experiment (HIRAX) is a radio interferometer array currently in development, with an initial 256-element array to be deployed at the South African Radio Astronomy Observatory Square Kilometer Array site in South Africa. Each of the 6 m, f / 0.23 dishes will be instrumented with dual-polarization feeds operating over a frequency range of 400 to 800 MHz. Through intensity mapping of the 21 cm emission line of neutral hydrogen, HIRAX will provide a cosmological survey of the distribution of large-scale structure over the redshift range of 0.775 < z < 2.55 over ∼15,000 square degrees of the southern sky. The statistical power of such a survey is sufficient to produce ∼7 % constraints on the dark energy equation of state parameter when combined with measurements from the Planck satellite. Additionally, HIRAX will provide a highly competitive platform for radio transient and HI absorber science while enabling a multitude of cross-correlation studies. We describe the science goals of the experiment, overview of the design and status of the subcomponents of the telescope system, and describe the expected performance of the initial 256-element array as well as the planned future expansion to the final, 1024-element array.