First hard X-ray detection of the non-thermal emission around the Arches
  cluster: morphology and spectral studies with NuSTAR
  The Arches cluster is a young, densely packed massive star cluster in our
Galaxy that shows a high level of star formation activity. The nature of the
extended non-thermal X-ray emission around the cluster remains unclear. The
observed bright Fe K_alpha line emission at 6.4 keV from material that is
neutral or in a low ionization state can be produced either by X-ray
photoionization or by cosmic-ray particle bombardment or both. In this paper we
report on the first detection of the extended emission around the Arches
cluster above 10 keV with the NuSTAR mission, and present results on its
morphology and spectrum. The spatial distribution of the hard X-ray emission is
found to be consistent with the broad region around the cluster where the 6.4
keV line is observed. The interpretation of the hard X-ray emission within the
context of the X-ray reflection model puts a strong constraint on the
luminosity of the possible illuminating hard X-ray source. The properties of
the observed emission are also in broad agreement with the low-energy
cosmic-ray proton excitation scenario.
