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

TL;DR

This paper reports initial results from the AARTFAAC Cosmic Explorer program, setting new upper limits on the 21-cm power spectrum during the EDGES absorption trough, using wide-field radio observations and advanced data processing.

Contribution

First application of the ACE pipeline to AARTFAAC data, providing the deepest upper limits on 21-cm fluctuations at redshift 17.9-18.6, and demonstrating the feasibility of detecting stronger signals.

Findings

Upper limits of $ ext{(8139 mK)}^2$ and $ ext{(8549 mK)}^2$ at $k=0.144h$ cMpc$^{-1}$ for two LST bins.

Combined upper limit of $ ext{(7388 mK)}^2$ at the same $k$ value, the deepest at these redshifts.

Validation of noise estimates and foreground subtraction methods in the data pipeline.

Abstract

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.