2025A&A...698A.186M

We present new upper limits on the 21 cm signal power spectrum from the epoch of reionisation (EoR), at redshifts z≈10.1,9.1, and 8.3, based on reprocessed observations from the Low-Frequency Array (LOFAR). The analysis incorporates significant enhancements in calibration methods, sky model subtraction, radio-frequency interference (RFI) mitigation, and an improved signal separation technique using machine learning to develop a physically motivated covariance model for the 21 cm signal. These advancements have markedly reduced previously observed excess power due to residual systematics, bringing the measurements closer to the theoretical thermal noise limit across the entire k-space. Using comparable observational data, we achieve a two- to fourfold improvement over our previous LOFAR limits, with best upper limits of $ \Delta _{21}^2 < (68.7\,{\mathrm {mK}})^2 $ at k=0.076 h cMpc^–1, $ \Delta _{21}^2 < (54.3\,{\mathrm {mK}})^2 $ at k=0.076 h cMpc^–1, and $ \Delta _{21}^2 < (65.5\,{\mathrm {mK}})^2 $ at k=0.083 h cMpc^–1 at redshifts z≈10.1,9.1, and 8.3, respectively. These new multi-redshift upper limits provide new constraints that can be used to refine our understanding of the astrophysical processes during the EoR. Comprehensive validation tests, including signal injection, were performed to ensure the robustness of our methods. The remaining excess power is attributed to residual foreground emissions from distant sources, beam model inaccuracies, and low-level RFI. We discuss ongoing and future improvements to the data processing pipeline aimed at further reducing these residuals, thereby enhancing the sensitivity of LOFAR observations in the quest to detect the 21 cm signal from the EoR.