2022ApJ...925...91S

Mergers between galaxy clusters often drive shocks into the intracluster medium, the effects of which are sometimes visible via temperature and density jumps in the X-ray and via radio emission from relativistic particles energized by the shock's passage. A2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev-Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow-up observations confirmed it to be a merging low-mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. Giant Metrewave Radio Telescope (GMRT) data in bands 2, 3, & 4 (covering 120-750 MHz) show an extended radio feature resembling a radio relic near the location of a temperature discontinuity in the X-rays. We measure a Mach number from the X-ray temperature jump ({{ \mathcal M }}_{X}=1.6±0.2). Several characteristics of radio relics (location and morphology of extended radio emission) are found in A2108, making this cluster one of the few low-mass mergers (M _L–M = 1.8 ± 0.4 x 10¹⁴ M _☉) likely hosting a radio relic. The radio spectrum is exceptionally steep (α = -2 at the lowest frequencies), and the radio power is very weak (P _1.4 GHz_ = 1 x 10²² W Hz^–1). To account for the shock/relic offset, we propose a scenario in which the shock created the relic by re-accelerating a cloud of pre-existing relativistic electrons and then moved away, leaving behind a fading relic. The electron-aging timescale derived from the high-frequency steepening in the relic spectrum is consistent with the shock travel time to the observed X-ray discontinuity. However, the lower flux in GMRT band 4 data causing the steepening could be due to instrumental limitations and deeper radio data are needed to constrain the spectral slope of the relic at high frequencies. A background cluster, 4' from the merger, may have contributed to the ROSAT and Planck signals, but SZ modeling shows that the merger system is still X-ray underluminous, supporting the use of this approach to identifying merger-disrupted clusters.