Ultra-low-frequency LOFAR spectral indices of cluster radio halos T. Pasini, F. De Gasperin, M. Brüggen, R. Cassano, A. Botteon, G. Brunetti, H. W. Edler, R. J. van Weeren, V. Cuciti, T. Shimwell et al. (5 more) A&A, 689 (2024) A218 Published online: 13 September 2024 DOI: 10.1051/0004-6361/202450697