Discovery of inverse-Compton X-ray emission and robust estimation of magnetic field in a galaxy group

Observed in a significant fraction of clusters and groups of galaxies, diffuse radio synchrotron emission reveals the presence of relativistic electrons and magnetic fields permeating large-scale systems. Although these non-thermal electrons are expected to upscatter cosmic microwave background photons up to hard X-ray energies, such inverse-Compton (IC) X-ray emission has so far not been unambiguously detected on cluster/group scales. Here we report the first robust ($4.6\,\sigma$) detection of extended IC X-ray emission from MRC$\,$0116+111, a group of galaxies. This unambiguous detection provides the most direct and least model-dependent estimate of the volume-averaged magnetic field within a galaxy group. Such estimates can serve as a fulcrum to theories of magnetic field generation within the largest gravitationally bound systems in the Universe.