Abstract
ABSTRACTThe H i 21 cm absorption line is masked by bright foregrounds and systematic distortions that arise due to the chromaticity of the antenna used to make the observation coupling to the spectral inhomogeneity of these foregrounds. We demonstrate that these distortions are sufficient to conceal the 21 cm signal when the antenna is not perfectly achromatic and that simple corrections assuming a constant spatial distribution of foreground power are insufficient to overcome them. We then propose a new physics-motivated method of modelling the foregrounds of 21 cm experiments in order to fit the chromatic distortions as part of the foregrounds. This is done by generating a simulated sky model across the observing band by dividing the sky into N regions and scaling a base map assuming a distinct uniform spectral index in each region. The resulting sky map can then be convolved with a model of the antenna beam to give a model of foregrounds and chromaticity parametrized by the spectral indices of the N regions. We demonstrate that fitting this model for varying N using a Bayesian nested sampling algorithm and comparing the results using the evidence allows the 21 cm signal to be reliably detected in data of a relatively smooth conical log spiral antenna. We also test a much more chromatic conical sinuous antenna and find this model will not produce a reliable signal detection, but in a manner that is easily distinguishable from a true detection.
Highlights
The details of the development of the universe between the epoch of recombination and the formation of modern structure are not well understood
Sponding Bfactor (ν) to attempt to correct for chromatic distortions as in Equation (14). This correction makes the assumption that the power distribution of the sky across the entire band is well approximated by that of the base map, and that the sky power scales with a uniform spectral index of 2.5
The ability to detect the HI 21cm signal from beneath the galactic and extra-galactic foregrounds is heavily dependent on the ability to accurately model systematic distortions in the data
Summary
The details of the development of the universe between the epoch of recombination and the formation of modern structure are not well understood This is because this intermediate period of the cosmic dark ages, cosmic dawn and epoch of reionisation is very difficult to detect directly. The first are interferometric instruments like the upcoming SKA (Dewdney et al 2009), as well as HERA (DeBoer et al 2017), PAPER (Parsons et al 2010), MWA (Lonsdale et al 2009) and LOFAR (van Haarlem et al 2013) These experiments are designed to detect the full spatially varying power spectrum of the 21cm signal. There are experiments designed to detect the spatially averaged “monopole” 21cm signal These “global” 21cm experiments include EDGES (Bowman et al 2008), SARAS (Patra et al 2013) and BIGHORNS (Sokolowski et al 2015). A similar systematic is identified by Bevins et al (2020)
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