We thank the anonymous referee for constructive comments that helped to improve this manuscript. FM thanks Soumyajit Mandal for fruitful discussions. The material is based upon work supported by NASA under award number 80GSFC21M0002. NW is supported by the GACR grant number21-13491X. M-LG-M acknowledges financial support from grant number RTI2018-096228-B-C31 (MCIU/AEI/FEDER,UE), from the coordination of the participation in SKA-SPAIN financed by the Ministry of Science and Innovation (MICIN), and from the State Agency for Research of the Spanish Ministry of Science, Innovation and Universities through the 'Center of Excellence Severo Ochoa' awarded to the Instituto de Astrofisica de Andalucia (SEV-2017-0709). G-K thanks the Indian National Science Academy for a Senior Scientist position. ARL was supported by the Gates Cambridge Scholarship, by the St John's College Benefactors' Scholarships, by NSERC through the Postgraduate Scholarship-Doctoral Programme (PGS D) under grant number PGSD3-535124-2019 and by FRQNT through the FRQNT Graduate Studies Research Scholarship-Doctoral level under grant number 274532. AS was supported by the Women In Science Excel (WISE) programme of the Netherlands Organization for Scientific Research (NWO), and acknowledges the World Premier Research Center Initiative (WPI) and the Kavli IPMU for the continued hospitality. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). SRON is supported financially by NWO.

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 the Giant Metrewave Radio Telescope (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. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society

Peer reviewed