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arXiv:1811.04917 [astro-ph.IM]AbstractReferencesReviewsResources

A Proof of Concept on Constraining the Foreground Spectrum for Global 21 cm Cosmology through Projection-induced Polarimetry

Bang D. Nhan, David D. Bordenave, Richard F. Bradley, Jack O. Burns, Patricia J. Klima, Keith Tauscher, David Rapetti

Published 2018-11-12Version 1

Detecting the cosmological sky-averaged (global) 21 cm spectrum as a function of observed frequency will provide a powerful tool to study the ionization and thermal history of the intergalactic medium (IGM) in the early Universe ($\sim$ 400 million years after the Big Bang). The biggest challenge in conventional ground-based total-power global 21 cm experiments is the removal of the foreground synchrotron emission ($\sim 10^3$-$10^4$ K) to uncover the weak cosmological signal (tens to hundreds of mK) due to corruptions on the spectral smoothness of the foreground spectrum by instrumental effects. An absorption profile has been reported recently at 78 MHz in the sky-averaged spectrum by the EDGES experiment. An independent approach is necessary to confirm that this observed feature is indeed the global 21 cm signal. In this paper, we present a new polarimetry-based observational approach that relies on the dynamic characteristics of the foreground emission at the circumpolar region to track and remove the foreground spectrum directly, without relying on generic foreground models as in conventional approaches. Due to asymmetry and the Earth's rotation, the projection of the anisotropic foreground sources onto a wide-view antenna pointing at the North Celestial Pole (NCP) induces a net polarization which varies with time in a unique twice-diurnal periodicity. Different from the total-power global 21 cm experiments, the Cosmic Twilight Polarimeter (CTP) is designed to measure and separate the varying foreground from the isotropic cosmological background simultaneously in the same observation. By combining preliminary results of the prototype instrument with numerical simulations, we evaluate the practicality and feasibility for implementing this technique to obtain an independent global 21 cm measurement in the near future using an upgraded CTP.

Comments: 14 pages, 19 figures, 1 table, submitting to ApJ
Categories: astro-ph.IM
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