Radio-relic and the diffuse emission trail discovered in a low mass
  galaxy cluster Abell 1697
  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
$M_{500}^{X-ray}=2.9^{+0.8}_{-0.7}\times10^{14}~\rm{M_{\odot}}$, 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
$\alpha(144,325,1400~\rm{MHz})=-0.98\pm0.01$ and magnetic field of
$B_{eq}\sim0.6~\mu$G. This relic, located in the northeast periphery of the
cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the
width ($\alpha_{144 \rm{MHz}}^{1.4\rm{GHz}}(inj)=-0.70\pm0.11$ to $\alpha_{144
\rm{MHz}}^{1.4\rm{GHz}}(edge)=-1.19\pm0.15$), as well as indications of a
co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of
the relic is $P_{1.4GHz}=8.5\pm1.1\times10^{23}~\rm{W\;Hz^{-1}}$, 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 ($790\times550$ kpc) diffuse radio emission towards the cluster
center, that is likely an ultra-steep spectrum ($\alpha_{144
\rm{MHz}}^{1.4\rm{GHz}}<-1.84$) 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 $B_{eq}\sim0.8~\mu$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.
