Revived Fossil Plasma Sources in Galaxy Clusters
  It is well established that particle acceleration by shocks and turbulence in
the intra-cluster medium can produce cluster-scale synchrotron emitting
sources. However, the detailed physics of these particle acceleration processes
is still not well understood. One of the main open questions is the role of
fossil relativistic electrons that have been deposited in the intra-cluster
medium by radio galaxies. These synchrotron-emitting electrons are very
difficult to study, as their radiative life time is only tens of Myrs at GHz
frequencies, and are therefore a relatively unexplored population. Despite the
typical steep radio spectrum due to synchrotron losses, these fossil electrons
are barely visible even at radio frequencies well below a GHz. However, when a
pocket of fossil radio plasma is compressed, it boosts the visibility at
sub-GHz frequencies, creating so-called radio phoenices. This compression can
be the result of bulk motion and shocks in the ICM due to merger activity. In
this paper, we demonstrate the discovery potential of low frequency radio sky
surveys to find and study revived fossil plasma sources in galaxy clusters. We
used the 150~MHz TGSS and 1.4 GHz NVSS sky surveys to identify candidate radio
phoenices. A subset of three candidates were studied in detail using deep
multi-band radio observations (LOFAR and GMRT), X-ray (\textit{Chandra} or
\textit{XMM-Newton}) and archival optical observations. Two of the three
sources are new discoveries. Using these observations, we identified common
observational properties (radio morphology, ultra-steep spectrum, X-ray
luminosity, dynamical state) that will enable us to identify this class of
sources more easily, and help to understand the physical origin of these
sources.
