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

Abstract

We present first results from radio observations with the Murchison Widefield
\nArray seeking to constrain the power spectrum of 21 cm brightness temperature
\nfluctuations between the redshifts of 11.6 and 17.9 (113 and 75 MHz). Three
\nhours of observations were conducted over two nights with significantly
\ndifferent levels of ionospheric activity. We use these data to assess the
\nimpact of systematic errors at low frequency, including the ionosphere and
\nradio-frequency interference, on a power spectrum measurement. We find that
\nafter the 1-3 hours of integration presented here, our measurements at the
\nMurchison Radio Observatory are not limited by RFI, even within the FM band,
\nand that the ionosphere does not appear to affect the level of power in the
\nmodes that we expect to be sensitive to cosmology. Power spectrum detections,
\ninconsistent with noise, due to fine spectral structure imprinted on the
\nforegrounds by reflections in the signal-chain, occupy the spatial Fourier
\nmodes where we would otherwise be most sensitive to the cosmological signal. We
\nare able to reduce this contamination using calibration solutions derived from
\nautocorrelations so that we achieve an sensitivity of 10 mK⁴ on comoving
\nscales k ≲ 0.5 h Mpc⁻¹. This represents the first upper limits on
\nthe 21 cm power spectrum fluctuations at redshifts 12 ≲ z ≲ 18 
\nbut is still limited by calibration systematics. While calibration improvements
\nmay allow us to further remove this contamination, our results emphasize that
\nfuture experiments should consider carefully the existence of and their ability
\nto calibrate out any spectral structure within the EoR window.