Integrated spectrum of the extended radio structure. The light blue and gray points denote the estimates of the diffuse flux computed by integrating over the regions with significance > 2.5σ or by fitting with Gaussian profiles the 0.4–2 kλ naturally weighted maps of each spectral band, respectively (see the main text in Sect. 3.1.2 for details). We use the same color scheme for the corresponding best-fit lines and 1σ confidence intervals. The dark blue point represents the integrated flux measured from the EMU image. All the open markers denote values that were not included in this analysis. For reference, we report in green the expected flux and corresponding uncertainties of a radio halo for a cluster of mass equal to the best-fit value reported in Kim et al. (2019) for SPT-CL J2106-5844 and following the radio power to mass scaling relation by Cuciti et al. (2021a) under the assumption of a spectral index α = −0.87 (Sect. 3.1.2).