## 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 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 Δ^{2}_{21} < (8139 mK)^{2} and Δ^{2}_{21} < (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 Δ^{2}_{21} < (7388 mK)^{2} at k = 0.144 hcMpc^{−1}. These are the deepest upper limits thus far at these redshifts.

Original language | English |
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Pages (from-to) | 4158-4173 |

Number of pages | 16 |

Journal | Monthly Notices of the Royal Astronomical Society |

Volume | 499 |

Issue number | 3 |

DOIs | |

State | Published - 1 Dec 2020 |

### Bibliographical note

Publisher Copyright:© 2020 The Author(s)

## Keywords

- Dark ages
- Diffuse radiation
- First stars
- Methods: data analysis
- Methods: statistical
- Radio lines: general
- Reionization
- Techniques: interferometric