The peculiar mini-halo in the Shapley Supercluster member Abell 3558

We present a multiband study of the diffuse emission in the galaxy cluster A3558, located in the core of the Shapley Supercluster. Using new MeerKAT (Meer Karoo Array Telescope) ultra-high-frequency (UHF) Band and uGMRT (Upgraded Giant Metrewave Radio Telescope) Band-3 observations, and published MeerKAT L-band and ASKAP (Australian Square Kilometre Array Pathfinder) 887 MHz data, we perform a detailed analysis of the diffuse emission in the cluster centre. We complement with XMM–Newton X-ray information for a thorough study of the connection between the thermal and non-thermal properties of the cluster. We find that the diffuse radio emission in the cluster centre is more extended than published earlier, with a previously undetected extension spanning 100 kpc towards the north beyond the innermost cold front, increasing the total size of the emission to 550 kpc, and shows a clear spatial correlation with the X-ray features. The overall radio spectrum is steep ((Formula presented) = 1.18 ± 0.10), with local fluctuations which show several connections with the X-ray surface brightness, cold fronts, and residual emission. The point-to-point correlation between the radio and X-ray surface brightness is sublinear, and steepens with increasing frequency. We discuss the classification of the diffuse emission considering its overall properties, those of the intracluster medium, and the existing scaling laws between the radio and X-ray quantities in galaxy clusters. We conclude that it is a mini-halo, powered by turbulent (re)-acceleration induced by sloshing motions within the cluster region delimited by the cold fronts, and it supports the picture of a known minor merger between A3558 and the group SC 1327–312 with mass ratio 5:1.