Evolutionary phases of merging clusters as seen by LOFAR
  Massive, merging galaxy clusters often host giant, diffuse radio sources that
arise from shocks and turbulence; hence, radio observations can be useful for
determining the merger state of a cluster. In preparation for a larger study,
we selected three clusters -- Abell 1319, Abell 1314, and RXC J1501.3+4220
(Z7215) -- making use of the new LOFAR Two-Metre Sky Survey (LoTSS) at 120-168
MHz, and together with archival data, show that these clusters appear to be in
pre-merging, merging, and post-merging states, respectively. We argue that
Abell 1319 is likely in its pre-merging phase, where three separate cluster
components are about to merge. There are no radio halos nor radio relics
detected in this system. Abell 1314 is a highly-disturbed, low-mass cluster
which is likely in the process of merging. This low-mass system does not show a
radio halo, however, we argue that the merger activates mechanisms that cause
electron re-acceleration in the large 800 kpc radio tail associated with
IC~711. In the cluster Z7215 we discover diffuse radio emission at the cluster
center, and we classify this emission as a radio halo, although it is dimmer
and smaller than expected by the radio halo power versus cluster mass
correlation. We suggest that the disturbed cluster Z7215 is in its post-merging
phase. Systematic studies of this kind over a larger sample of clusters
observed with LoTSS will help constrain the time scales involved in turbulent
re-acceleration and the subsequent energy losses of the underlying electrons.
