A Study of the Dynamic Polarization Induced by the Galactic Foreground with the Cosmic Twilight Polarimeter.
Abstract
Recent efforts in the study of the Cosmic Dawn, the period when the first luminous sources formed in the early Universe occurring ~400 million years after the Big Bang, have faced great challenges in instrumental and observational methods. Observations of this period are focused on the detection of the highly redshifted (30 > z > 15) HI 21-cm emission that probes the ionization and thermal history of the primordial Intergalactic Medium. However, the greatest challenge affecting both global sky-averaged and interferometric experiments is the removal of the strong galactic synchrotron foreground that is expected to be around 4-5 orders of magnitude greater than the cosmological 21-cm signal ( ~10s - 100s of mK). Additionally, certain instrumental effects that are difficult to characterize (e.g. beam chromaticity, ground screen excitation modes, noise parameter estimation for the receiver, etc) would further corrupt the intrinsically-smooth foreground power spectrum and hinder the extraction of the weak background 21-cm signal. In light of these problems we are continuing development on the Cosmic Twilight Polarimeter (CTP), a prototype global 21-cm experiment designed to study and understand the combined nature of the signal, foreground, instrument, and environment through physically motivated effects and modeling. Current efforts for the CTP have focused on understanding the spectral and dynamical behavior of the sky-antenna system through observations of induced polarization due to the off-boresight projection of foreground sources onto the antenna plane. Simulations of this induced polarization signal show both linear and circular polarization components that are excited with an unpolarized but spatially anisotropic source distribution and that this effect is sensitive to spectral and spatial variations in the sky and antenna beam. Preliminary observations with a zenith pointed CTP prototype at Green Bank,WV. have detected non-zero linear and circular polarization signals that are periodic in sidereal time implying that these signals are potentially generated by the dynamic interaction between the sky and antenna. Further work is required to improve instrument calibrations and to develop more realistic simulations of this effect to determine its utility for future global 21-cm experiments.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23517521B