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Mega-parsec scale radio sources in the form of halos and relics are oftendetected in dynamically disturbed galaxy clusters. Although they are associatedwith merger-induced turbulence and shocks, respectively, their formation is notfully understood. We aim to identify the mechanisms responsible for particleacceleration and magnetic field amplification in the halo and relics of thegalaxy cluster ClG 0217+70. We observed ClG 0217+70 with LOFAR at 141 MHz andVLA at 1.5 GHz, and combine these observations with VLA 1.4 GHz archival datato study the morphological and spectral properties of the diffuse sources. Weadd Chandra archival data to examine the thermal and non-thermal properties ofthe halo. Our LOFAR and VLA data confirm the presence of a giant radio halo inthe cluster centre and multiple relics in the outskirts. The radio and X-rayemission from the halo are correlated, implying a tight relation between thethermal and non-thermal components. The diffuse radio structure in the southeast with a projected size of 3.5 Mpc is the most extended radio relic detectedto date. The spectral index across the relic width steepens towards the clustercentre, suggesting the electron ageing in the post-shock regions. The shockMach numbers for the relics derived from the spectral index map range between2.0 and 3.2. However, the integrated spectral indices lead to increasingly highMach numbers for the relics farther from the cluster centre. This discrepancycould be because the relation between injection and integrated spectra does nothold for distant shocks, suggesting that the cooling time for theradio-emitting electrons is longer than the crossing time of the shocks. Thevariations in the surface brightness of the relics and the low Mach numbersimply that the radio-emitting electrons are re-accelerated from fossil plasmathat could originate in active galactic nuclei.

A 3.5 Mpc-long radio relic in the galaxy cluster ClG 0217+70