Observational consistency and future predictions for a 3.5 keV ALP to
  photon line
  Motivated by the possibility of explaining the 3.5 keV line through dark
matter decaying to axion-like particles that subsequently convert to photons,
we study ALP-photon conversion for sightlines passing within 50 pc of the
galactic centre. Conversion depends on the galactic centre magnetic field which
is highly uncertain. For fields at low or mid-range of observational estimates
(10--100 $\mu$G), no observable signal is possible. For fields at the high
range of observational estimates (a pervasive poloidal mG field over the
central 150 pc) it is possible to generate sufficient signal to explain recent
observations of a 3.5 keV line in the galactic centre. In this scenario, the
galactic centre line signal comes predominantly from the region with $z > 20$
pc, reconciling the results from the Chandra and XMM-Newton X-ray telescopes.
The dark matter to ALP to photon scenario also naturally predicts the
non-observation of the 3.5 keV line in stacked galaxy spectra. We further
explore predictions for the line flux in galaxies and suggest a set of galaxies
that is optimised for observing the 3.5 keV line in this model.
