A simulation-calibrated limit on the H i power spectrum from the GMRT Epoch of Reionization experiment

The Giant Metrewave Radio Telescope Epoch of Reionization experiment is an ongoing effort to measure the power spectrum from neutral hydrogen at high redshift. We have previously reported an upper limit of (70 mK)2 at wavenumbers of k 0.65 h Mpc−1 using a basic piecewise-linear foreground subtraction. In this paper, we explore the use of a singular value decomposition to remove foregrounds with fewer assumptions about the foreground structure. Using this method, we also quantify, for the first time, the signal loss due to the foreground filter and present new power spectra adjusted for this loss, providing a revised measurement of a 2σ upper limit at (248 mK)2 for k = 0.50 h Mpc−1. While this revised limit is larger than previously reported, we believe it to be more robust and still represents the best current constraint on reionization at z 8.6.