A Cosmic ALP Background and the Cluster Soft X-ray Excess in A665, A2199
  and A2255
  It has been proposed that an excess in soft X-ray emission observed from many
galaxy clusters can be explained by conversion into photons of axion-like
particles (ALPs) in the cluster's magnetic field. Previously it has been shown
that conversion of this primordially-generated background of relativistic
ALPs---a cosmic ALP background (CAB)---can explain the observed soft X-ray
excess in both the centre and the outskirts of the Coma cluster. Here we extend
this work to the three clusters A665, A2199 and A2255. We use a stochastic
model of the cluster magnetic field to numerically calculate ALP-photon
conversion probabilities to predict the CAB-generated soft X-ray flux in these
clusters. We compare this flux to ROSAT PSPC observations of the three
clusters, and use these observations to constrain the CAB parameter space. We
find the CAB can reproduce the magnitude of the observed excess in A2199 and
A2255 for the same CAB parameters that match the observed soft excess in the
Coma cluster. We also find good fit to the morphology of the excesses in these
clusters. Simulation of CAB conversion in the cluster A665 is in mild tension
with the other clusters due to producing a small but observable excess at large
radii where none is observed. This tension is alleviated considering the
uncertainty on predicting the count rate in the ROSAT detector, and on the
systematics affecting the magnetic field determination. Overall we find good
agreement between the CAB parameters for the four clusters studied so far.
