First limits on the 21 cm power spectrum during the Epoch of X-ray heating

We present first results from radio observations with the Murchison Widefield Array seekingto constrain the power spectrum of 21 cm brightness temperature fluctuations between theredshifts of 11.6 and 17.9 (113 and 75 MHz). 3 h of observations were conducted over twonights with significantly different levels of ionospheric activity. We use these data to assess theimpact of systematic errors at low frequency, including the ionosphere and radio-frequencyinterference, on a power spectrum measurement. We find that after the 1–3 h of integrationpresented here, our measurements at the Murchison Radio Observatory are not limited by RFI,even within the FM band, and that the ionosphere does not appear to affect the level of power inthe modes that we expect to be sensitive to cosmology. Power spectrum detections, inconsistentwith noise, due to fine spectral structure imprinted on the foregrounds by reflections in thesignal-chain, occupy the spatial Fourier modes where we would otherwise be most sensitive tothe cosmological signal. We are able to reduce this contamination using calibration solutionsderived from autocorrelations so that we achieve an sensitivity of 104 mK on comoving scalesk 0.5 h Mpc−1. This represents the first upper limits on the 21 cm power spectrum fluctuationsat redshifts 12 z 18 but is still limited by calibration systematics. While calibrationimprovements may allow us to further remove this contamination, our results emphasize thatfuture experiments should consider carefully the existence of and their ability to calibrate outany spectral structure within the EoR window.