Abell 1430: A merging cluster with exceptional diffuse radio emission
  Diffuse radio emission has been found in many galaxy clusters, predominantly
in massive systems which are in the state of merging. The radio emission can
usually be classified as relic or halo emission, which are believed to be
related to merger shocks or volume-filling turbulence, respectively. Recent
observations have revealed radio bridges for some pairs of very closeby galaxy
clusters. The mechanisms that may allow to explain the high specific density of
relativistic electrons, necessary to explain the radio luminosity of these
bridge regions, are poorly explored. We analyse the galaxy cluster Abell 1430
with LoTSS data in detail and complement it with recent JVLA L-band
observations, XMM-Newton, Chandra, and SDSS data. Moreover, we compare our
results to clusters extracted from the "The Three Hundred Project" cosmological
simulation. We find that Abell 1430 consists of two components, namely A1430-A
and A1430-B. We speculate that the two components undergo an off-axis merger.
The more massive component shows diffuse radio emission which can be classified
as radio halo showing a low radio power given the mass of the cluster. Most
interestingly, there is extended diffuse radio emission, dubbed as the
`Pillow', which is apparently related to A1430-B and thus related to low
density intracluster or intergalactic medium. To date, a only few examples for
emission originating from such regions are known. These discoveries are crucial
to constrain possible acceleration mechanisms, which may allow to explain the
presence of relativistic electrons in these regions. Our results indicate a
spectral index of $\alpha_{144\,\text{MHz}}^{1.5\,\text{GHz}}=-1.4\pm0.5$ for
the Pillow. If future observations confirm a slope as flat as the central value
of -1.4 or even flatter, this would pose a severe challenge for the electron
acceleration scenarios.
