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

## TL;DR

This study presents X-ray and radio observations of the galaxy group MRC 0116+111, revealing an unusually high radio-to-X-ray luminosity ratio, a large extent of radio emission, and a high magnetic field, indicating complex AGN activity and intragroup medium interactions.

## Contribution

First detailed comparison of thermal and non-thermal components in MRC 0116+111, highlighting its unique radio and X-ray properties and magnetic field estimates.

## Key findings

- Radio-to-X-ray luminosity ratio is among the highest observed.
- Radio emission extends about three times further than X-ray emission.
- Magnetic field lower limit is at least 4.3 μG.

## Abstract

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 ($\sim$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-energising 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 ($\ge$4.3 $\mu$G). Finally, we discuss some interesting properties of a distant ($z \simeq 0.525$) galaxy cluster serendipitously discovered in our EPIC field of view.