The unusually weak and exceptionally steep radio relic in Abell 2108
  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.
