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 seeking to constrain the power spectrum of 21 cm brightness temperature between the redshifts of 11. 6 and 17. 9 (113 and 75 MHz). Three of observations were conducted over two nights with significantly levels of ionospheric activity. We use these data to assess the of systematic errors at low frequency, including the ionosphere and -frequency interference, on a power spectrum measurement. We find that the 1-3 hours of integration presented here, our measurements at the Radio Observatory are not limited by RFI, even within the FM band, that the ionosphere does not appear to affect the level of power in the that we expect to be sensitive to cosmology. Power spectrum detections, with noise, due to fine spectral structure imprinted on the by reflections in the signal-chain, occupy the spatial Fourier where we would otherwise be most sensitive to the cosmological signal. We able to reduce this contamination using calibration solutions derived from so that we achieve an sensitivity of 10 mK⁴ on comoving k ≲ 0. 5 h Mpc⁻¹. This represents the first upper limits on 21 cm power spectrum fluctuations at redshifts 12 ≲ z ≲ 18 is still limited by calibration systematics. While calibration improvements allow us to further remove this contamination, our results emphasize that experiments should consider carefully the existence of and their ability calibrate out any spectral structure within the EoR window.