Hidden axion dark matter decaying through mixing with QCD axion and the 3.5 keV X-ray line

Understand

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.