Multi-frequency studies of radio relics in the galaxy clusters A4038,
  A1664 and A786
  We present a multi-frequency study of radio relics associated with the galaxy
clusters A4038, A1664 and A786. Radio images, integrated spectra, spectral
index maps and fits to the integrated spectra in the framework of the adiabatic
compression model are presented. Images of the relic in A4038 at 150, 240 and
606 MHz with the Giant Metrewave Radio Telescope have revealed extended
ultra-steep spectrum (\alpha ~ -1.8 to -2.7) emission of an extent 210 x 80
kpc^2. The model of passively evolving radio lobes compressed by a shock best
fits the integrated spectrum. The relic with a circular morphology at the
outskirts of the cluster A1664 has an integrated spectral index of -1.10+/-0.06
and is best fit by the model of radio lobes lurking for ~ 4x10^7 yr. The relic
near A786 has a curved spectrum and is best fit by a model of radio lobes
lurking for ~ 3x10^7 yr. At 4.7 GHz, a compact radio source, possibly the
progenitor of the A786 relic, is detected near the center of the radio relic.
The A786 radio relic is thus likely a lurking radio galaxy rather than a site
of cosmological shock as has been considered in earlier studies.
