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We present new high sensitivity observations of the radio relic in A521carried out with the Giant Metrewave Radio Telescope at 327 MHz and with theVery Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequenciesand carried out a detailed spectral analysis, based on the integrated radiospectrum between 235 MHz and 4.9 GHz, and on the spectral index image in thefrequency range 327-610 MHz. To this aim we used the new GMRT observations andother proprietary as well as archival data. We also searched for a possibleshock front co-located with the relic on a short archival Chandra X-rayobservation of the cluster. The integrated spectrum of the relic is consistentwith a single power law; the spectral index image shows a clear trend ofsteepening going from the outer portion of the relic toward the cluster centre.We discuss the origin of the source in the light of the theoretical models forthe formation of cluster radio relics. Our results on the spectral propertiesof the relic are consistent with acceleration of relativistic electrons by ashock in the intracluster medium. This scenario is further supported by ourfinding of an X-ray surface brightness edge coincident with the outer border ofthe radio relic. This edge is likely a shock front.

Shock acceleration as origin of the radio relic in A521?