Another shock for the Bullet cluster, and the source of seed electrons
  for radio relics
  With Australia Telescope Compact Array observations, we detect a highly
elongated Mpc-scale diffuse radio source on the eastern periphery of the Bullet
cluster 1E0657-55.8, which we argue has the positional, spectral and
polarimetric characteristics of a radio relic. This powerful relic (2.3+/-0.1 x
10^25 W Hz^-1) consists of a bright northern bulb and a faint linear tail. The
bulb emits 94% of the observed radio flux and has the highest surface
brightness of any known relic. Exactly coincident with the linear tail we find
a sharp X-ray surface brightness edge in the deep Chandra image of the cluster
-- a signature of a shock front in the hot intracluster medium (ICM), located
on the opposite side of the cluster to the famous bow shock. This new example
of an X-ray shock coincident with a relic further supports the hypothesis that
shocks in the outer regions of clusters can form relics via diffusive shock
(re-)acceleration. Intriguingly, our new relic suggests that seed electrons for
reacceleration are coming from a local remnant of a radio galaxy, which we are
lucky to catch before its complete disruption. If this scenario, in which a
relic forms when a shock crosses a well-defined region of the ICM polluted with
aged relativistic plasma -- as opposed to the usual assumption that seeds are
uniformly mixed in the ICM -- is also the case for other relics, this may
explain a number of peculiar properties of peripheral relics.
