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 K 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, which employs the AARTFAAC wide-field image, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range z = 17.9-18.6 (Δν = 72.36-75.09 MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5-23.75 and 23.75-24.00 h, 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 2σ upper limits on the 21-cm power spectrum of Δ²₂₁ < (8139 mK)² and Δ²₂₁ < (8549 mK)² at k = 0.144 hcMpc⁻¹ for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of Δ²₂₁ < (7388 mK)² at k = 0.144 hcMpc⁻¹. These are the deepest upper limits thus far at these redshifts.