A $\mathcal{M}\gtrsim3$ shock in `El Gordo' cluster and the origin of
  the radio relic
  We present an X-ray and radio study of the famous `El Gordo', a massive and
distant ($z=0.87$) galaxy cluster. In the deep (340 ks) Chandra observation,
the cluster appears with an elongated and cometary morphology, a sign of its
current merging state. The GMRT radio observations at 610 MHz reveal the
presence of a radio halo which remarkably overlaps the X-ray cluster emission
and connects a couple of radio relics. We detect a strong shock
($\mathcal{M}\gtrsim3$) in the NW periphery of the cluster, co-spatially
located with the radio relic. This is the most distant ($z=0.87$) and one of
the strongest shock detected in a galaxy cluster. This work supports the
relic-shock connection and allows to investigate the origin of these radio
sources in a uncommon regime of $\mathcal{M}\gtrsim3$. For this particular case
we found that shock acceleration from the thermal pool is still a viable
possibility.
