Cluster X-ray line at $3.5\,{\rm keV}$ from axion-like dark matter

Understand

The recently reported X-ray line signal at $E_\gamma \simeq 3.5\, {\rm keV}$ from a stacked spectrum of various galaxy clusters and the Andromeda galaxy may be originating from a decaying dark matter particle of the mass $2 E_\gamma$.

• A light axion-like scalar is suggested as a natural candidate for dark matter and its production mechanisms are closely examined.
• We show that the right amount of axion relic density with the preferred parameters, $m_a \simeq 7 \,{\rm keV}$ and $f_a \simeq 4\times 10^{14}\, {\rm GeV}$, can be naturally obtainable from the decay of inflaton.
• If the axions were produced from the saxion decay, it could not have constituted the total relic density due to the bound from structure formation.