Magnetic fields in the outskirts of galaxy clusters: Insights from radio relics

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Under some circumstances, shock waves produced by the collision of galaxy clusters accelerate electrons to relativistic velocities to form so-called radio relics. Relics are polarised (generally at a level of 10 to 30 %) sources with a steep spectral index. They have elongated shapes, with sizes between 100 and 1000 kpc, and are usually located at the cluster periphery. Some observations suggest that relics host relatively large magnetic fields, with typical values of a few µG. These fields are derived using several methods, mostly equipartition arguments, comparison of hard X-ray and radio emission, the spectral aging and the width of relics seen edge-on. Here we propose a combination of LOFAR polarimetry observations with magneto-hydrodynamical simulations to investigate the origins of these fields. Relics thus become an important probe of the outer regions of galaxy clusters. This field has recently attracted a lot of attention as a flurry of X-ray and Sunyaev-Zel’dovich (SZ) observations have shown signatures of significant non-thermal pressure components in some peripheral cluster regions. With the expected discovery of many 100s new radio relics with LOFAR, we can thus get unprecedented access to these unknown areas of the Universe.