The Epoch of Reionization 21 cm Bispectrum at z = 8.2 from MWA Data. I. Foregrounds and Preliminary Upper Limits

Abstract

We attempt to measure the z = 8.2 Epoch of Reionization (EoR) 21 cm bispectrum (BS) using Murchison Widefield Array 154.2 MHz data. We find that B(k1⊥, k2⊥, k3⊥, k1∥, k2∥), the 3D cylindrical BS, exhibits a foreground wedge, similar to P(k1⊥, k1∥), the 21 cm cylindrical power spectrum. However, the BS foreground wedge, which depends on (k1⊥, k1∥), (k2⊥, k2∥), and (k3⊥, k3∥), the three sides of a triangle, is more complicated. Considering various foreground avoidance scenarios, we identify the region where all three sides are outside the foreground wedge as the EoR window for the 21 cm BS. However, the EoR window is contaminated by a periodic pattern of spikes that arises from the periodic pattern of missing frequency channels in the data. We evaluate the binned 3D spherical BS for triangles of all possible sizes and shapes, and present results for Δ3, the mean-cube brightness temperature fluctuations. The best 2σ upper limits we obtain for the EoR 21 cm signal are UL = (1.81 × 10 ) mK 3 3 3 3 at k1 = 0.008 Mpc−1 and UL = (2.04 × 10 ) mK 3 3 3 3 at k1 = 0.012 Mpc−1 for equilateral and squeezed triangles, respectively. These are foreground dominated and are many orders of magnitude larger than the predicted EoR 21 cm signal (∼103 mK^3).