A Massive Cluster at z = 0.288 Caught in the Process of Formation: The
  Case of Abell 959
  The largest galaxy clusters are observed still to be forming through major
cluster-cluster mergers, often showing observational signatures such as radio
relics and giant radio haloes. Using LOFAR Two-meter Sky Survey data, we
present new detections of both a radio halo (with a spectral index of
$\alpha_{143}^{1400}=1.48^{+0.06}_{-0.23}$) and a likely radio relic in Abell
959, a massive cluster at a redshift of z=0.288. Using a sample of clusters
with giant radio haloes from the literature (80 in total), we show that the
radio halo in A959 lies reasonably well on the scaling relations between the
thermal and non-thermal power of the system. Additionally, we find evidence
that steep-spectrum haloes tend to reside in clusters with high X-ray
luminosities relative to those expected from cluster LM scaling relations,
indicating that such systems may preferentially lie at an earlier stage of the
merger, consistent with the theory that some steep-spectrum haloes result from
low-turbulence mergers. Lastly, we find that halo systems containing radio
relics tend to lie at lower X-ray luminosities, relative to those expected from
cluster LM scaling relations, for a given halo radio power than those without
relics, suggesting that the presence of relics indicates a later stage of the
merger, in line with simulations.
