Diffuse radio emission in the galaxy cluster SPT-CL J2031-4037: a steep
  spectrum intermediate radio halo?
  The advent of sensitive low frequency radio observations has revealed a
number of diffuse radio objects with peculiar properties that are challenging
our understanding about the physics of the intracluster medium. Here, we report
the discovery of a steep spectrum radio halo surrounding the central Brightest
Cluster Galaxy (BCG) in the galaxy cluster SPT-CL J2031-4037. This cluster is
morphologically disturbed yet has a weak cool core, an example of cool
core/non-cool core transition system, which harbours a radio halo of $\sim 0.7$
Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to
that in the 325 MHz observation, and the spectral index of the part of the halo
visible at 325 MHz to 1.7 GHz frequencies was found to be $-1.35 \pm 0.07$.
Also, $P_{1.4\ \mathrm{GHz}}$ was found to be $0.77 \times 10^{24}$ W Hz$^{-1}$
which falls in the region where radio mini-halos, halo upper limits and
ultra-steep spectrum (USS) halos are found in the $P_{1.4\ \mathrm{GHz}} -
L_\mathrm{X}$ plane. Additionally, simulations presented in the paper provide
support to the scenario of the steep spectrum. The diffuse radio emission found
in this cluster may be a steep spectrum "intermediate" or "hybrid" radio halo
which is transitioning into a mini-halo.
