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Hidden axions may be coupled to the standard model particles through akinetic or mass mixing with QCD axion. We study a scenario in which a hiddenaxion constitutes a part of or the whole of dark matter and decays into photonsthrough 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 forthe QCD axion decay constant at an intermediate scale, if the mixing issufficiently small. In such a two component dark matter scenario, theprimordial density perturbations of the hidden axion can be highlynon-Gaussian, leading to a possible dispersion in the X-ray line strength fromvarious galaxy clusters and near-by galaxies. We also discuss how the paralleland orthogonal alignment of two axions affects their couplings to gauge fields.

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