[Paper Review] The Hydrogen Intensity and Real-time Analysis eXperiment: 256-Element Array Status and Overview

Devin Crichton, Moumita Aich+98 more|arXiv (Cornell University)|Sep 28, 2021

Radio Astronomy Observations and TechnologyPhysics and Astronomy73 references69 citations

TL;DR

HIRAX proposes a 256-element radio interferometer array at the South African SKA site, using 6m f/0.23 dishes with dual-polarization feeds to map 21 cm emission from neutral hydrogen over 15,000 deg² of the southern sky (0.775 < z < 2.55). The survey enables ~7% constraints on the dark energy equation of state when combined with Planck data, while also supporting transient and absorber science via high-sensitivity, wideband intensity mapping with advanced digital backend systems including F- and X-engines.

ABSTRACT

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 (SARAO) Square Kilometer Array (SKA) site in South Africa. Each of the 6m, $f/0.23$ dishes will be instrumented with dual-polarisation feeds operating over a frequency range of 400-800 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 $\sim$15,000 square degrees of the southern sky. The statistical power of such a survey is sufficient to produce $\sim$7 percent 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. In this paper, we describe the science goals of the experiment, overview of the design and status of the sub-components 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.

Motivation & Objective

• To develop a high-sensitivity, wideband radio interferometer array for cosmological intensity mapping of neutral hydrogen in the post-reionization era.
• To achieve sub-arcminute pointing accuracy and sub-millimeter receiver positioning precision to minimize beam and systematics errors.
• To enable cosmological constraints on dark energy with ~7% precision by combining HIRAX data with Planck measurements.
• To provide a competitive platform for transient and HI absorber science through wideband, real-time data processing.
• To lay the foundation for a future 1024-element array through phased deployment and system validation.

Proposed method

• Deploying 256 dual-polarization 6 m f/0.23 dishes across a 100 m × 100 m array at the SARAO SKA site in South Africa.
• Using RF front-ends and digital backend systems (F-engine and X-engine) to process signals in real time across 400–800 MHz bandwidth.
• Implementing a geometric error budget with target tolerances: 0.5 mm receiver position, 2.5′ beam alignment, 1 mm dish surface deviation, and 1′ boresight/elevation axis orthogonality.
• Conducting electromagnetic simulations using CST Studio Suite to model beam shape degradation from receiver misalignment and surface deviations.
• Performing beam characterization via drone-based and holographic measurements at HartRAO, Dominion Radio Astrophysical Observatory, and Green Bank Observatory.
• Validating the X-engine system with on-sky data at Bleien Observatory, Switzerland, prior to full deployment.

Experimental results

Research questions

• RQ1 Can a 256-element intensity mapping array achieve the required photometric and astrometric stability for cosmological power spectrum measurements at z ≈ 1–2.5?
• RQ2 What mechanical and RF system tolerances are necessary to maintain beam shape and sensitivity across a wide bandwidth and array configuration?
• RQ3 To what extent can HIRAX improve constraints on the dark energy equation of state when combined with Planck data?
• RQ4 How effectively can HIRAX detect and characterize radio transients and HI absorbers in the 400–800 MHz band?
• RQ5 What is the performance of the F- and X-engine digital backend systems in real-time, wideband signal processing for intensity mapping?

Key findings

• The 256-element HIRAX array is designed to survey 15,000 square degrees of the southern sky at redshifts 0.775 < z < 2.55, covering the post-reionization epoch.
• The instrument is expected to deliver ~7% constraints on the dark energy equation of state parameter when combined with Planck satellite data.
• Mechanical tolerances are tightly controlled: receiver position within 0.5 mm, beam alignment within 2.5′, and dish surface deviations within 1 mm.
• The F-engine and X-engine digital backend systems are fully designed and prototyped, with one X-engine node already deployed for on-sky testing.
• Beam characterization using drone and holographic methods confirms alignment and beam shape fidelity within design specifications.
• The telescope mechanical design has been finalized, with initial prototype dishes expected to be assembled and tested in 2022.