2020A&A...633A..59P

We report the discovery of a putative radio relic, 830 kpc in length, and found towards the outskirts of galaxy cluster Abell 1697 (z=0.181), using the LOFAR Two Meter Sky Survey at 144MHz. An X-ray-inferred mass of M₅₀₀^X–ray=2.9_–0.7^+0.8x10¹⁴ M_☉ places Abell 1697 among the least massive relic hosts. The relic is also detected at 325MHz in the Westerbork Northern Sky Survey and at 1.4GHz in the NRAO VLA Sky Survey, with an average spectral index of α(144, 325, 1400MHz)=-0.98±0.01, and magnetic field of B_eq∼0.6µG. This relic, located in the north-east periphery of the cluster, is 300kpc wide, exhibits a gradual spectral steepening across the width (α_144MHz^1.4GHz(inj)=-0.70±0.11 to α_144MHz^1.4GHz(edge)= 1.19±0.15), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P_1.4GHz=8.5±1.1x10²³W/Hz, which is found to be in good agreement with the expected empirical correlation between the radio power and largest linear size of relics. The relic is trailed by extended (790x550kpc) diffuse radio emission towards the cluster centre, which is likely an ultra-steep spectrum (α_144MHz^1.4GHz< -1.84) radio source. This structure is also found to be older by at least 190Myr, has a very low surface brightness of 0.3µJy/arcsec² and magnetic field B_eq∼0.8µG, similar to that of a radio phoenix. Finally, we discuss the possible mechanisms responsible for the relic and the trailing diffuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil electrons in the remnant radio lobes of active galactic nuclei by the cluster merger shocks.