J021659-044920: a relic giant radio galaxy at z ~ 1.3
  We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at
redshift $z \sim 1.3$ that exhibits large-scale extended, nearly co-spatial,
radio and X-ray emission from radio lobes, but no detection of Active Galactic
Nuclei core, jets and hotspots. The total angular extent of the GRG at the
observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is
found to be ${\sim}$ 2.4 arcmin, that corresponds to a total projected linear
size of $\sim$ 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz
shows high spectral curvature (${\alpha}_{\rm 0.610~GHz}^{\rm 1.4~GHz} -
{\alpha}_{\rm 0.240~GHz}^{\rm 0.325~GHz}$ $>$ 1.19) with sharp steepening above
0.325 GHz, consistent with relic radio emission that is $\sim$ 8 $\times$
10$^{6}$ yr old. The radio spectral index map between observed frame 0.325 and
1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from
outer-end to inner-end, indicating backflow of plasma in the lobes. The
extended X-ray emission characterized by an absorbed power-law with photon
index $\sim$ 1.86 favours inverse-Compton scattering of the Cosmic Microwave
Background (ICCMB) photons as the plausible origin. Using both X-ray and radio
fluxes under the assumption of ICCMB we estimate the magnetic field in the
lobes to be 3.3 $\mu$G. The magnetic field estimate based on energy
equipartition is $\sim$ 3.5 $\mu$G. Our work presents a case study of a rare
example of a GRG caught in dying phase in the distant Universe.
