## 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

Funding Information:BKG and LVEK acknowledge the financial support from a NOVA cross-network grant. BKG acknowledges the National Science Foundation grant AST-1836019. FGM, MK, and AS acknowledge support from a SKA-NL roadmap grant from the Dutch Ministry of OCW. This work was supported in part by ERC grant 247295 'AARTFAAC' to RAMJW. LOFAR, the Low Frequency Array designed and constructed by ASTRON, has facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy. This research made use of publicly available software developed for LOFAR and AARTFAAC telescopes. Here is a list of software packages used in the analysis: AARTFAAC2MS (https://github.com/aroffringa/aartfaac2ms), AOFLAGGER (https://gitlab.com/aroffringa/ao flagger), DPPP (https://github.com/lofar-astron/DP3), WSCLEAN (ht tps://gitlab.com/aroffringa/wsclean), and GPR foreground removal code (https://gitlab.com/flomertens/ps eor). The analysis also relies on the PYTHON programming language (https://www.python.org) and several publicly available PYTHON software modules: ASTROPY (https://www.astropy.org; The Astropy Collaboration 2013), GPY (https://github.com/SheffieldML/GPy), EMCEE (https://emcee.readthedocs.io/en/stable/; Foreman-Mackey et al. 2013), MATPLOTLIB (https://matplotlib.org/), SCIPY (https://www.scipy.org/), and NUMPY (https://numpy.org/).

Publisher Copyright:

© 2020 The Author(s)

## Keywords

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