Discovery of inverse-Compton X-ray emission and estimate of the volume-averaged 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 of galaxies. 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. Using deep, new proprietary XMM-Newton observations (~200 ks of clean exposure), we report a 4.6 σ detection of extended IC X-ray emission in MRC 0116 +111, an extraordinary group of galaxies at z = 0.131. Assuming a spectral slope derived from low-frequency radio data, the detection remains robust to systematic uncertainties. Together with low-frequency radio data from GMRT, this detection provides an estimate for the volume-averaged magnetic field of (1.9 ± 0.3) μG within the central part of the group. This value can serve as an anchor for studies of magnetic fields in the largest gravitationally bound systems in the Universe.