We report the discovery of a putative radio relic , 830 kpc in length and found toward the outskirts of galaxy cluster Abell 1697 ( z = 0.181 ) , using the LOFAR Two Meter Sky Survey ( LoTSS ) at 144 MHz .
With an X-ray-inferred mass of M _ { 500 } ^ { X - ray } = 2.9 ^ { +0.8 } _ { -0.7 } \times 10 ^ { 14 } ~ { } { M _ { \odot } } , this places Abell 1697 among the least massive relic hosts .
The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey ( WENSS ) and at 1.4 GHz in the NRAO VLA Sky Survey ( NVSS ) with an average spectral index of \alpha ( 144 , 325 , 1400 ~ { } MHz ) = -0.98 \pm 0.01 and magnetic field of B _ { eq } \sim 0.6 ~ { } \mu G. This relic , located in the northeast periphery of the cluster , is 300 kpc wide , exhibits a gradual spectral steepening across the width ( \alpha _ { 144 MHz } ^ { 1.4 GHz } ( inj ) = -0.70 \pm 0.11 to \alpha _ { 144 MHz } ^ { 1.4 GHz } ( edge ) = -1.19 \pm 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 _ { 1.4 GHz } = 8.5 \pm 1.1 \times 10 ^ { 23 } ~ { } { W Hz ^ { -1 } } , which is found to be in good agreement with the expected empirical correlation between the radio power and Largest Linear Size ( LLS ) of relics .
The relic is trailed by extended ( 790 \times 550 kpc ) diffuse radio emission towards the cluster center , that is likely an ultra-steep spectrum ( \alpha _ { 144 MHz } ^ { 1.4 GHz } < -1.84 ) radio source .
This structure is also found to be older by at least 190 Myrs , has a very low surface brightness of 0.3 ~ { } \mu Jy arcsec ^ { -2 } and magnetic field B _ { eq } \sim 0.8 ~ { } \mu G , 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 from an old AGN activity by the cluster merger shocks .