A LOFAR study of non-merging massive galaxy clusters
  Centrally located diffuse radio emission has been observed in both merging
and non-merging galaxy clusters. Depending on their morphology and size, we
distinguish between giant radio haloes, which occur predominantly in merging
clusters, and mini haloes, which are found in non-merging, cool-core clusters.
Low-frequency sensitive observations are required to assess whether the
emission discovered in these few cases is common in galaxy clusters or not.
With this aim, we carried out a campaign of observations with the LOw Frequency
ARay (LOFAR) in the frequency range 120 - 168 MHz of nine massive clusters
selected from the \textit{Planck} SZ catalogue, which had no sign of major
mergers. In this paper, we discuss the results of the observations that have
led to the largest cluster sample studied within the LOFAR Two-metre Sky
Survey, and we present Chandra X-ray data used to investigate the dynamical
state of the clusters, verifying that the clusters are currently not undergoing
major mergers, and to search for traces of minor or off-axis mergers. We
discover large-scale steep-spectrum emission around mini haloes in the
cool-core clusters PSZ1G139 and RXJ1720, which is not observed around the mini
halo in the non-cool-core cluster A1413. We also discover a new 570 kpc-halo in
the non-cool-core cluster RXCJ0142. We derived new upper limits to the radio
power for clusters in which no diffuse radio emission was found, and we discuss
the implication of our results to constrain the cosmic-ray energy budget in the
ICM. We conclude that radio emission in non-merging massive clusters is not
common at the sensitivity level reached by our observations and that no clear
connection with the cluster dynamical state is observed. Our results might
indicate that the sloshing of a dense cool core could trigger particle
acceleration on larger scales and generate steep-spectrum radio emission.
