Discovery of inverse-Compton X-ray emission and estimate of the volume-averaged magnetic field in a galaxy group

TL;DR

This paper reports the first significant detection of inverse-Compton X-ray emission in a galaxy group, allowing estimation of its magnetic field strength, which advances understanding of non-thermal processes in large-scale cosmic structures.

Contribution

The study provides the first unambiguous detection of IC X-ray emission in a galaxy group and estimates its magnetic field, using deep XMM-Newton and GMRT radio data.

Findings

Detected extended IC X-ray emission at 4.6σ in galaxy group MRC 0116+111

Estimated volume-averaged magnetic field of (1.9 ± 0.3) μG in the group

Confirmed the presence of relativistic electrons and magnetic fields in galaxy groups

Abstract

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 ($\sim$200 ks of clean exposure), we report a 4.6$\sigma$ 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 $(1.9 \pm 0.3)$ $\mu$G 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.