Context : Hierarchical models of structure formation predict that galaxy clusters grow via mergers of smaller clusters and galaxy groups , as well as through continuous accretion of gas . MACS~J0717.5+3745 is an X-ray luminous and complex merging cluster , located at a redshift of 0.5548 .
The cluster is suspected to host a bright radio relic , but up until now no detailed radio observations have been reported .
Here we present Giant Metrewave Radio Telescope ( GMRT ) radio observations at 610 MHz of this cluster .

Aims : The main aim of the observations is to study the diffuse radio emission within the galaxy cluster MACS~J0717.5+3745 related to the ongoing merger .

Methods : We have carried out GMRT 610 MHz continuum observations of MACS~J0717.5+3745 .
These are complemented by Very Large Array ( VLA ) archival observations at 1.4 , 4.9 and 8.5 GHz .

Results : We have discovered a radio halo in the cluster MACS~J0717.5+3745 with a size of about 1.2 Mpc .
The monochromatic radio power at 1400 MHz ( P _ { 1.4 } ) is 5 \times 10 ^ { 25 } W Hz ^ { -1 } , which makes it the most powerful radio halo known till date .
A 700 kpc radio structure , which we classify as a radio relic , is located in between the merging substructures of the system .
The global spectral index of radio emission within the cluster is found to be -1.24 \pm 0.05 between 4.9 GHz and 610 MHz .
We derive a value of 5.8 \mu G for the equipartition magnetic field strength at the location of the radio halo .
The location of the relic roughly coincides with regions of the intra-cluster medium ( ICM ) that have a significant enhancement in temperature as shown by Chandra .
The major axis of the relic is also roughly perpendicular to the merger axis .
This shows that the relic might be the result of a merger-related shock wave , where particles are accelerated via the diffuse shock acceleration ( DSA ) mechanism .
Alternatively , the relic might trace an accretion shock of a large-scale galaxy filament to the south-east .

Conclusions :