The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum
  in the EDGES absorption trough
  The 21-cm absorption feature reported by the EDGES collaboration is several
times stronger than that predicted by traditional astrophysical models. If
genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm
signal on degree scales (up to 1~Kelvin in rms), allowing these fluctuations to
be detectable in nearly 50~times shorter integration times compared to previous
predictions. We commenced the "AARTFAAC Cosmic Explorer" (ACE) program, that
employs the AARTFAAC wide-field imager, to measure or set limits on the power
spectrum of the 21-cm fluctuations in the redshift range $z = 17.9-18.6$
($\Delta\nu = 72.36-75.09$~MHz) corresponding to the deep part of the EDGES
absorption feature. Here, we present first results from two LST bins:
23.5-23.75h and 23.5-23.75h, each with 2~h of data, recorded in `semi
drift-scan' mode. We demonstrate the application of the new ACE data-processing
pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe
that noise estimates from the channel and time-differenced Stokes~$V$
visibilities agree with each other. After 2~h of integration and subtraction of
bright foregrounds, we obtain $2\sigma$ upper limits on the 21-cm power
spectrum of $\Delta_{21}^2 < (8139~\textrm{mK})^2$ and $\Delta_{21}^2 <
(8549~\textrm{mK})^2$ at $k = 0.144~h\,\textrm{cMpc}^{-1}$ for the two LST
bins. Incoherently averaging the noise bias-corrected power spectra for the two
LST bins yields an upper limit of $\Delta_{21}^2 < (7388~\textrm{mK})^2$ at $k
= 0.144~h\,\textrm{cMpc}^{-1}$. These are the deepest upper limits thus far at
these redshifts.
