Radio halos in galaxy clusters: the puzzling case of Abell 2142.

Clusters of galaxies allow us to probe the interplay between thermal and non-thermal components of the intra-cluster medium (ICM). Turbulence induced by mergers of clusters in the ICM can re-accelerate aged particles and trigger the formation of diffuse synchrotron radio sources. These provide information on the energy transfer mechanisms and magnetic fields on cosmic scales. Diffuse sources in the form of giant radio halos (RHs) and mini-halos (MHs) are found in major merging clusters and weakly perturbed relaxed systems, respectively. Despite this classification, a handful of hybrid RH-MH sources of unclear origin are known. Abell 2142 (A2142) is a puzzling cluster that exhibits intermediate properties between classical major merging and relaxed systems. It was known to host a peculiar hybrid halo. The unprecedentedly high sensitivity of LOFAR below 200 MHz allowed us to detect a third elusive radio component, which makes A2142 a unique case in the field of diffuse sources. We present the non-thermal properties of the three-component hybrid halo and its thermal ICM counterpart, and discuss possible scenarios that triggered re-acceleration processes on different scales.