[Paper Review] Measuring the 21 cm Global Brightness Temperature Spectrum During the Dark Ages with the SCI-HI Experiment

J. B. Peterson, Tabitha Voytek+3 more|arXiv (Cornell University)|Sep 9, 2014

Radio Astronomy Observations and Technology9 references3 citations

TL;DR

The SCI-HI experiment measures the global 21 cm brightness temperature during the Dark Ages using a dual-antenna system with HIbiscus designs centered at 70 MHz and 100 MHz, achieving residuals of 5–10 K after 4.4 hours of integration—below 1% of foregrounds—demonstrating feasibility for detecting the faint 21 cm signal despite RFI and systematics.

ABSTRACT

We present an update on the SCI-HI experiment, which is designed to measure the all-sky (global) 21 cm brightness temperature during the end of the Dark Ages. Results from preliminary observations in June 2013 are discussed, along with system improvements and planned future work.

Motivation & Objective

• To detect the global 21 cm brightness temperature signal during the Dark Ages, a period marking the transition from neutral hydrogen to the first stars.
• To overcome challenges from strong terrestrial radio frequency interference (RFI) by deploying to remote, radio-quiet islands.
• To develop and test a dual-antenna system with high coupling efficiency and wide bandwidth to capture the full spectral structure of the 21 cm signal.
• To reduce residuals below 100 mK through improved calibration and system stability, enabling detection of the 300 mK peak-to-peak 21 cm signal.
• To validate data processing pipelines using simulation-based calibration checks to ensure signal integrity and minimize systematic errors.

Proposed method

• Deployed a dual-antenna system using scaled HIbiscus designs (centered at 70 MHz and 100 MHz) with >90% coupling efficiency across 55–140 MHz, enabling wideband coverage.
• Used overlapping antenna bands (70–90 MHz) to cross-validate data and improve spectral consistency.
• Collected data over 9 days at Isla Guadalupe, Mexico, a remote biosphere reserve with minimal RFI, to reduce interference from terrestrial signals.
• Applied a radiometer equation-based analysis to estimate thermal noise and distinguish it from residual systematics and foregrounds.
• Implemented a simulation calibration technique that injected synthetic 21 cm signals into data to verify signal retention, confirming ~75% of added signal remained in residuals.
• Used daily LST-binned integration and RMS error estimation to assess day-to-day stability and data quality.

Experimental results

Research questions

• RQ1 Can a single-site, dual-antenna 21 cm experiment achieve residuals below 10 K after 4.4 hours of integration in a remote, low-RFI environment?
• RQ2 To what extent do self-generated RFI and power supply instability contribute to systematic errors in the 21 cm brightness temperature measurement?
• RQ3 How well does the data processing pipeline preserve the 21 cm signal, as verified by injecting simulated signals and measuring residual signal retention?
• RQ4 Can the observed residuals be attributed primarily to residual foregrounds or to instrumental systematics, and how do they compare to theoretical expectations?
• RQ5 Does deployment to even more isolated islands (e.g., Isla Socorro and Clarión) significantly reduce RFI and improve sensitivity to the 21 cm signal?

Key findings

• After 4.4 hours of integration, the SCI-HI experiment achieved residuals of 5–10 K, which is less than 1% of the foreground brightness temperature (>1000 K).
• The simulation calibration test confirmed that approximately 75% of an injected 21 cm signal was preserved in the residuals, indicating robust signal retention in the data pipeline.
• Self-generated RFI was identified as a dominant source of systematics, particularly above 90 MHz, and was found to be time-varying and frequency-dependent.
• Power supply instability from 12 V lead-acid batteries caused frequent data gaps and contributed to calibration errors, especially during incomplete 24-hour observation cycles.
• Despite the presence of residual RFI and systematics, the residual levels were lower than early predictions based on spectral index structure of foregrounds, indicating a favorable environment for future detection.
• Future deployments to Isla Socorro and Isla Clarión are expected to reduce RFI to below thermal noise levels, enabling a broader frequency range and improved sensitivity to the 21 cm signal.