Upper limits on the 21 cm power spectrum from the epoch of reionization

In this Thesis work, we analyse 21cm line observations taken with the Precision Array to Probe the Epoch of Reionization (PAPER) in the 120-180MHz range (6<z<10). We focus on the power spectrum estimate of the Epoch of Reionization (EoR) signal at four different redshift intervals from 8 hours of data. The analysis of PAPER data is unconventional: the array is constituted by 128 dipoles arranged in a redundant configuration in order to maximize the power spectrum sensitivity on a limited number of k-modes. For this reason, PAPER observations are not ideal to image and model foreground emission: we instead adopted a strategy that relies on avoidance, isolating the power spectrum region where the foreground emission is expected to be fainter than the EoR signal. We estimated the power spectrum from PAPER observations using only 30m baselines, the most numerous one. After an initial data selection, we developed a pipeline to estimate the power spectrum of the 21cm signal. The pipeline averages calibrated data over the duration of the observation, Fourier ("delay") transforms them along the frequency axis and then calculate cross-products between different baselines of the same length. The resulting cross-spectra are averaged to obtain a final estimate for each redshift interval. The delay transform separates the foregrounds, that are confined to the low k-modes. Higher k-modes are consistent with the noise power spectrum that we derived from the data themselves. We, therefore, estimated upper limits on the 21cm power spectrum. Finally, using a simple, analytic model of the 21cm signal, we calculated a lower limit on the IGM temperature at z=7.48.