Abstract
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. Nonetheless, the saxion decay is an interesting possibility, because the $3.5\, {\rm keV}$ line and dark radiation can be addressed simultaneously, being consistent with the Planck data. Small misalignment angles of the axion, ranging between $\theta_a\sim 10^{-4} -10^{-1}$ depending on the reheating temperature, can also be the source of axion production. The model with axion misalignment can satisfy the constraints for structure formation and iso-curvature perturbation.
Highlights
It has been recently reported by two groups that there exists a line signal at 3.5 keV in a stacked X-ray spectrum of galaxy clusters and the Andromeda galaxy [1,2]
We proposed a simple model for the keV-scale axion dark matter whose decay product into monochromatic photons can be the source of the recently reported X -ray spectrum at about 3.5 keV
Such a light axion can be produced from the decay of a heavy particle or from the coherent oscillation of the axion caused by misalignment
Summary
It has been recently reported by two groups that there exists a line signal at 3.5 keV in a stacked X-ray spectrum of galaxy clusters and the Andromeda galaxy [1,2]. We propose an axion-like particle as the source of the X -ray line at 3.5 keV.. The mass spectra of the axion-like particles can span a wide range, covering the keV scale. We may identify one of them as the axion-like particle explaining the X-ray line.. We show that an axion-like DM, produced preferably in the decay of the inflaton, can provide a good fit to the observed X-ray line by its decay into a pair of photons through anomaly interactions with the decay constant, fa 4 × 1014 GeV. 2, we describe our model for an axion-like DM and discuss the cosmological bounds from structure formation and isocurvature perturbation in Sect.
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