First evidence of diffuse ultra-steep-spectrum radio emission
  surrounding the cool core of a cluster
  Diffuse synchrotron radio emission from cosmic-ray electrons is observed at
the center of a number of galaxy clusters. These sources can be classified
either as giant radio halos, which occur in merging clusters, or as mini halos,
which are found only in cool-core clusters. In this paper, we present the first
discovery of a cool-core cluster with an associated mini halo that also shows
ultra-steep-spectrum emission extending well beyond the core that resembles
radio halo emission. The large-scale component is discovered thanks to LOFAR
observations at 144 MHz. We also analyse GMRT observations at 610 MHz to
characterise the spectrum of the radio emission. An X-ray analysis reveals that
the cluster is slightly disturbed, and we suggest that the steep-spectrum radio
emission outside the core could be produced by a minor merger that powers
electron re-acceleration without disrupting the cool core. This discovery
suggests that, under particular circumstances, both a mini and giant halo could
co-exist in a single cluster, opening new perspectives for particle
acceleration mechanisms in galaxy clusters.
