Detection of diffuse inverse-Compton X-ray emission in a galaxy group

At the dawn of the high-energy astrophysics era, the origin of the bright, extended X-ray emission seen towards clusters and groups of galaxies was debated. The advent of dedicated space observatories established that these systems shine in X-rays via thermal bremsstrahlung and line emission from a hot (10^7-8 K), magnetized, collisionally ionized medium permeating them. However, radio wavelength detections of diffuse synchrotron radiation from relativistic electrons in some systems implies the existence of a non-thermal component that should also boost cosmic microwave background (CMB) photons up to X-ray energies. Hitherto, such inverse-Compton (IC) emission has not been unambiguously detected on cluster/group scales. In this talk, we report a robust detection of X-ray IC emission in a large-scale gravitationally bound system - the group of galaxies MRC 0116+111. We will see how this detection provides a unique unbiased estimate of the volume-average magnetic field intensity within this group. Such a unrivaled accuracy in the magnetic field strength is a pre-requisite for a realistic understanding of processes underlying the magnetic genesis within the largest gravitationally bound structures in our Universe.