Abstract
ABSTRACT Detection of millikelvin-level signals from the ‘Cosmic Dawn’ requires an unprecedented level of sensitivity and systematic calibration. We report the theory behind a novel calibration algorithm developed from the formalism introduced by the EDGES collaboration for use in 21-cm experiments. Improvements over previous approaches are provided through the incorporation of a Bayesian framework and machine learning techniques such as the use of Bayesian evidence to determine the level of frequency variation of calibration parameters that is supported by the data, the consideration of correlation between calibration parameters when determining their values, and the use of a conjugate-prior based approach that results in a fast algorithm for application in the field. In self-consistency tests using empirical data models of varying complexity, our methodology is used to calibrate a 50 Ω ambient-temperature load. The RMS error between the calibration solution and the measured temperature of the load is 8 mK, well within the 1 σ noise level. Whilst the methods described here are more applicable to global 21-cm experiments, they can easily be adapted and applied to other applications, including telescopes such as HERA and the SKA.
Highlights
For nearly a century, scientists have been using radio-frequency instruments to advance the study of astronomy and complement information from the visual regime of the electromagnetic spectrum (Pritchard & Loeb 2012)
We present a novel calibration algorithm that improves on the work of the Experiment to Detect the Global EoR Signature (EDGES) team (Rogers & Bowman 2012) through the utilisation of a Bayesian framework to promote efficient use of the data to remove systematics
We presented the development of a calibration methodology based on the procedure used by EDGES but with key improvements to characterise reflections arising at connections within the receiver
Summary
Scientists have been using radio-frequency instruments to advance the study of astronomy and complement information from the visual regime of the electromagnetic spectrum (Pritchard & Loeb 2012). The signal, centred at 78 MHz with a width corresponding to a period between 180 million and 270 million years after the Big Bang, matches the theoretical position in frequency, but its depth of ∼ 0.5 K is a factor of two greater than the largest predictions from theoretical models (Cohen et al 2017) This discrepancy would suggest that the temperature difference between the IGM and the cosmic microwave background was much larger than previously thought and would require new physics to explain, such as dark matter-baryon interactions (Barkana 2018) or excess radio backgrounds (Ewall-Wice et al 2020).
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