PAPER-64 Constraints on Reionization: The 21 cm Power Spectrum at z = 8.4

In this paper, we report new limits on 21cm emission from cosmic reionization based on a 135 day observing campaign with a 64-element deployment of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) in South Africa. This work extends the work presented in Parsons et al. (2014) with more collecting area, a longer observing period, improved redundancybased calibration, improved fringe-rate ltering, and updated power-spectral analysis using optimal quadratic estimators. The result is a new 2 upper limit on 2 (k) of (22.4 mK) 2 in the range 0:15 < k < 0:5h Mpc 1 at z = 8:4. This represents a three-fold improvement over the previous best upper limit. As we discuss in more depth in a forthcoming paper (Pober et al. 2015), this upper limit supports and extends previous evidence against extremely cold reionization scenarios. We conclude with a discussion of implications for future 21cm reionization experiments, including the newly funded Hydrogen Epoch of Reionization Array. Subject headings: