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

Detecting the cosmological global (sky-averaged) 21 cm signal 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 greatest challenge in conventional 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) since the intrinsic smoothness of the foreground spectrum is corrupted instrumental effects. Although the EDGES team has recently reported an absorption profile at 78 MHz in the sky-averaged spectrum, it is necessary to confirm this detection with an independent approach. In this paper, we present a new, polarimetry-based, approach which 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 anisotropic foreground sources onto a wide-view antenna pointing at the North Celestial Pole (NCP) induces a net polarization with distinctive temporal variations. Different from the total-power approach, the Cosmic Twilight Polarimeter (CTP) is designed to measure and separate the varying foreground from the isotropic cosmological background simultaneously. 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.