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 seeking to constrain the power spectrum of 21 cm brightness temperature
fluctuations between the redshifts of 11.6 and 17.9 (113 and 75 MHz). Three
hours of observations were conducted over two nights with significantly
different levels of ionospheric activity. We use these data to assess the
impact of systematic errors at low frequency, including the ionosphere and
radio-frequency interference, on a power spectrum measurement. We find that
after the 1-3 hours of integration presented 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 in the
modes that we expect to be sensitive to cosmology. Power spectrum detections,
inconsistent with noise, due to fine spectral structure imprinted on the
foregrounds by reflections in the signal-chain, occupy the spatial Fourier
modes where we would otherwise be most sensitive to the cosmological signal. We
are able to reduce this contamination using calibration solutions derived from
autocorrelations so that we achieve an sensitivity of $10^4$ mK on comoving
scales $k\lesssim 0.5 h$Mpc$^{-1}$. This represents the first upper limits on
the $21$ cm power spectrum fluctuations at redshifts $12\lesssim z \lesssim 18$
but is still limited by calibration systematics. While calibration improvements
may allow us to further remove this contamination, our results emphasize that
future experiments should consider carefully the existence of and their ability
to calibrate out any spectral structure within the EoR window.
