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The 21-cm absorption feature reported by the EDGES collaboration is severaltimes stronger than that predicted by traditional astrophysical models. Ifgenuine, a deeper absorption may lead to stronger fluctuations on the 21-cmsignal on degree scales (up to 1~Kelvin in rms), allowing these fluctuations tobe detectable in nearly 50~times shorter integration times compared to previouspredictions. We commenced the "AARTFAAC Cosmic Explorer" (ACE) program, thatemploys the AARTFAAC wide-field imager, to measure or set limits on the powerspectrum 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 EDGESabsorption 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 `semidrift-scan' mode. We demonstrate the application of the new ACE data-processingpipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observethat noise estimates from the channel and time-differenced Stokes~$V$visibilities agree with each other. After 2~h of integration and subtraction ofbright foregrounds, we obtain $2\sigma$ upper limits on the 21-cm powerspectrum 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 LSTbins. Incoherently averaging the noise bias-corrected power spectra for the twoLST 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 atthese redshifts.

The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum in the EDGES absorption trough