Dark Matter Detection with Hard X-ray Telescopes
  We analyze the impact of future hard X-ray observations on the search for
indirect signatures of particle dark matter in large extragalactic systems such
as nearby clusters or groups of galaxies. We argue that the hard X-ray energy
band falls squarely at the peak of the inverse Compton emission from electrons
and positrons produced by dark matter annihilation or decay for a large class
of dark matter models. Specifically, the most promising are low-mass models
with a hard electron-positron annihilation final state spectrum and
intermediate-mass models with a soft electron-positron spectrum. We find that
constraints on dark matter models similar to the current constraints from the
Fermi Gamma-Ray Space Telescope will be close to the sensitivity limit of the
near-term hard X-ray telescopes NuSTAR and ASTRO-H for relatively long
observations. An instrument like the Wide Field Imager (WFI) proposed for
ATHENA would instead give a significant gain in sensitivity to dark matter if
placed in a low background orbit similar to NuSTAR's; however, given the higher
expected background level for ATHENA's proposed orbit at L2, its sensitivity
will be similar to that of NuSTAR.
