Improving 21-cm Epoch of Reionization Power Spectrum Limits by Characterizing and Mitigating Radio Frequency Interference

Abstract

The Epoch of Reionization (EoR) is the period in which the universe's hydrogen transformed from being predominantly neutral to being predominantly ionized. An extremely promising way to explore the EoR is by making use of the 21-cm line from the hyperfine transition in neutral Hydrogen. Ultimately we seek 3D maps of the intergalactic medium throughout this transition. However, preliminary efforts will focus on constraining the spatial power spectra of these maps at various cosmological redshifts. Redshifted 21-cm radiation can be observed using modern radio telescopes. The expected signal from the EoR is extremely faint. There are numerous systematic effects that, left unchecked, will totally dominate the EoR signal in the power spectrum. One of these effects, which is the primary subject of this thesis, is radio frequency interference (RFI). RFI is essentially any unwanted non-astrophysical signal observed by the instrument. We develop a new method for finding RFI in radio interferometers, called \textsc{SSINS}, which finds significantly fainter RFI than previous methods. Based on semi-analytic calculations and simulations, we place limits on total allowable apparent RFI flux density in the final power spectrum integration at roughly 1 mJy for relatively optimistic EoR models. We also explore the effects of RFI flagging on power spectrum measurements, finding that it can produce deleterious excess power in much the same way that observed RFI does. These results suggest that current RFI flagging strategies must be totally reconsidered to preserve the dynamic range of the power spectrum measurement. By examining a season of data from the Murchison Widefield Array, we expose residual RFI undetected by \textsc{SSINS} that contaminates power spectra at a meaningful level. Contaminated power spectra usually contain observations that already contain flags from \textsc{SSINS}, demonstrating the capacity for \textsc{SSINS} to improve power spectrum measurements. Finally, we draw an upper limit on the EoR power spectrum. Our lowest upper limit is $\Delta^2 \leq 1.61\cdot10^4 \text{ mK}^2$ at $k=0.258\text{ h Mpc}^{-1}$ at a redshift of 7.1.