  Mergers between galaxy clusters often drive shocks into the intra cluster
medium (ICM), 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. Abell2108 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 covering 120-750MHz 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. Several characteristics of radio relics are found in
Abell2108, making this cluster one of the few low mass mergers likely hosting a
radio relic. The radio spectrum is exceptionally steep, and the radio power is
very weak (P1.4GHz=1E22W/Hz). 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.
