A 3.55 keV Photon Line and its Morphology from a 3.55 keV ALP Line
  Galaxy clusters can efficiently convert axion-like particles (ALPs) to
photons. We propose that the recently claimed detection of a 3.55--3.57 keV
line in the stacked spectra of a large number of galaxy clusters and the
Andromeda galaxy may originate from the decay of either a scalar or fermionic
$7.1$ keV dark matter species into an axion-like particle (ALP) of mass $m_{a}
\lesssim 6\cdot 10^{-11}~{\rm eV}$, which subsequently converts to a photon in
the cluster magnetic field. In contrast to models in which the photon line
arises directly from dark matter decay or annihilation, this can explain the
anomalous line strength in the Perseus cluster. As axion-photon conversion
scales as $B^2$ and cool core clusters have high central magnetic fields, this
model can also explains the observed peaking of the line emission in the cool
cores of the Perseus, Ophiuchus and Centaurus clusters, as opposed to the much
larger dark matter halos. We describe distinctive predictions of this scenario
for future observations.
