  We analyzed a deep Chandra observation (334 ks) of the galaxy cluster Abell
115 and detected a shock co-spatial with the radio relic. The X-ray surface
brightness profile across the shock region presents a discontinuity,
corresponding to a density compression factor $\mathcal{C}=2.0\pm0.1$ leading
to a Mach number $\mathcal{M}=1.7\pm0.1$ ($\mathcal{M}=1.4-2$ including
systematics). Temperatures measured in the upstream and downstream regions are
consistent with what expected for such a shock:
$T_u=4.3^{+1.0}_{-0.6}\:\rm{keV}$ and $T_d=7.9^{+1.4}_{-1.1}\:\rm{keV}$
respectively, implying a Mach number $\mathcal{M}=1.8^{+0.5}_{-0.4}$. So far,
only few other shocks discovered in galaxy clusters are consistently detected
from both density and temperature jumps. The spatial coincidence between this
discontinuity and the radio relic edge strongly supports the view that shocks
play a crucial role in powering these synchrotron sources. We suggest that the
relic is originated by shock re-acceleration of relativistic electrons rather
than acceleration from the thermal pool. The position and curvature of the
shock and the associated relic are consistent with an off-axis merger with
unequal mass ratio where the shock is expected to bend around the core of the
less massive cluster.
