Discovery of a giant radio fossil in the Ophiuchus galaxy cluster
  The Ophiuchus galaxy cluster exhibits a curious concave gas density
discontinuity at the edge of its cool core. It was discovered in the Chandra
X-ray image by Werner and collaborators, who considered a possibility of it
being a boundary of an AGN-inflated bubble located outside the core, but
discounted this possibility because it required much too powerful an AGN
outburst. Using low-frequency (72-240 MHz) radio data from MWA GLEAM and GMRT,
we found that the X-ray structure is, in fact, a giant cavity in the X-ray gas
filled with diffuse radio emission with an extraordinarily steep radio
spectrum. It thus appears to be a very aged fossil of the most powerful AGN
outburst seen in any galaxy cluster ($pV\sim 5\times 10^{61}$ erg for this
cavity). There is no apparent diametrically opposite counterpart either in
X-ray or in the radio. It may have aged out of the observable radio band
because of the cluster asymmetry. At present, the central AGN exhibits only a
weak radio source, so it should have been much more powerful in the past to
have produced such a bubble. The AGN is currently starved of accreting cool gas
because the gas density peak is displaced by core sloshing. The sloshing itself
could have been set off by this extraordinary explosion if it had occurred in
an asymmetric gas core. This dinosaur may be an early example of a new class of
sources to be uncovered by low-frequency surveys of galaxy clusters.
