Discovery of a double radio relic in ZwCl1447.2+2619: A rare testbed for
  shock acceleration models with a peculiar surface brightness ratio
  We report a discovery of a double radio relic in the cluster merger
ZwCl1447.2+2619 ($z=0.376$) with uGMRT observations at $420\rm~MHz$ and
$700\rm~MHz$. The linear sizes of the northern and southern relics are
$\sim0.3~$Mpc and $\sim1.2~$Mpc, respectively, which is consistent with the
theoretical expectation that a larger relic is produced in the less massive
subcluster side. However, ZwCl1447.2+2619 is unlike other known double radio
relic systems, where the larger relics are much more luminous by several
factors. In this merger the higher surface brightness of the smaller northern
relic makes its total radio luminosity comparable to that of the much larger
southern relic. The surface brightness ratio $\sim0.1$ between the two radio
relics differs significantly from the relation observed in other double radio
relic systems. From our radio spectral analysis, we find that both relics
signify similar weak shocks with Mach numbers of $2.9\pm0.8$ and $2.0\pm0.7$
for the northern and southern relics, respectively. Moreover, the northern
relic is connected to a discrete radio source with an optical counterpart,
which indicates the possible presence of cosmic ray injection and
re-acceleration. Therefore, we propose that this atypical surface brightness
ratio can be explained with the particle acceleration efficiency precipitously
dropping in the weak shock regime and/or with re-acceleration of fossil cosmic
rays. Our multi-wavelength analysis and numerical simulation suggest that
ZwCl1447.2+2619 is a post-merger, which has experienced a near head-on
collision $\sim0.7\rm~Gyr$ ago.
