Mpc-scale diffuse radio emission in two massive cool-core clusters of
  galaxies
  Radio halos are diffuse synchrotron sources on scales of ~1 Mpc that are
found in merging clusters of galaxies, and are believed to be powered by
electrons re-accelerated by the merger-driven turbulence. We present
measurements of extended radio emission on similarly large scales in two
clusters of galaxies hosting cool cores: Abell 2390 and Abell 2261. The
analysis is based on interferometric imaging with the JVLA, VLA and GMRT. We
present detailed radio images of the targets, subtract the compact emission
components, and measure the spectral indices for the diffuse components. The
radio emission in A2390 extends beyond a known sloshing-like brightness
discontinuity, and has a very steep in-band spectral slope at 1.5 GHz that is
similar to some known ultra-steep spectrum radio halos. The diffuse signal in
A2261 is more extended than in A2390 but has lower luminosity. X-ray
morphological indicators, derived from XMM-Newton X-ray data, place these
clusters in the category of relaxed or regular systems, although some
asymmetric features that can indicate past minor mergers are seen in the X-ray
brightness images. If these two Mpc-scale radio sources are categorized as
giant radio halos, they question the common assumption of radio halos occurring
exclusively in clusters undergoing violent merging activity, in addition to
commonly used criteria in distinguishing between radio halos and mini-halos.
