Hidden axion dark matter decaying through mixing with QCD axion and the
  3.5 keV X-ray line
  Hidden axions may be coupled to the standard model particles through a
kinetic or mass mixing with QCD axion. We study a scenario in which a hidden
axion constitutes a part of or the whole of dark matter and decays into photons
through the mixing, explaining the 3.5 keV X-ray line signal. Interestingly,
the required long lifetime of the hidden axion dark matter can be realized for
the QCD axion decay constant at an intermediate scale, if the mixing is
sufficiently small. In such a two component dark matter scenario, the
primordial density perturbations of the hidden axion can be highly
non-Gaussian, leading to a possible dispersion in the X-ray line strength from
various galaxy clusters and near-by galaxies. We also discuss how the parallel
and orthogonal alignment of two axions affects their couplings to gauge fields.
