Magnetic fields and extraordinarily bright radio emission in the X-ray faint galaxy group MRC 0116 + 111

MRC 0116 + 111 is a nearby (z = 0.132) poor galaxy group, which was previously known for exhibiting a bright diffuse radio emission with no central point-like source, presumably related to a past activity of the active galactic nucleus (AGN) in its central cD galaxy. Here, we present an X-ray observation (∼30 ks of cleaned XMM-Newton/EPIC exposure) of this system, allowing us for the first time a detailed comparison between the thermal and non-thermal components of its intragroup medium (IGrM). Remarkably, we find that the radio-to-X-ray luminosity ratio is among the highest ever observed for a diffuse extragalactic source so far, while the extent of the observed radio emission is about three times larger than its observed soft X-ray emission. Although powerful AGN activity may have disturbed the dynamics of the thermal IGrM in the form of turbulence, possibly re-energizing part of the relativistic electron population, the gas properties lie within the L_X-T scaling relation established previously for other groups. The upper limit we find for the non-thermal inverse-Compton X-ray emission translates into a surprisingly high lower limit for the volume-averaged magnetic field of the group (≥4.3 μG). Finally, we discuss some interesting properties of a distant (z ≃ 0.525) galaxy cluster serendipitously discovered in our EPIC field of view.