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It has been proposed that an excess in soft X-ray emission observed from manygalaxy clusters can be explained by conversion into photons of axion-likeparticles (ALPs) in the cluster's magnetic field. Previously it has been shownthat conversion of this primordially-generated background of relativisticALPs---a cosmic ALP background (CAB)---can explain the observed soft X-rayexcess in both the centre and the outskirts of the Coma cluster. Here we extendthis work to the three clusters A665, A2199 and A2255. We use a stochasticmodel of the cluster magnetic field to numerically calculate ALP-photonconversion probabilities to predict the CAB-generated soft X-ray flux in theseclusters. We compare this flux to ROSAT PSPC observations of the threeclusters, and use these observations to constrain the CAB parameter space. Wefind the CAB can reproduce the magnitude of the observed excess in A2199 andA2255 for the same CAB parameters that match the observed soft excess in theComa cluster. We also find good fit to the morphology of the excesses in theseclusters. Simulation of CAB conversion in the cluster A665 is in mild tensionwith the other clusters due to producing a small but observable excess at largeradii where none is observed. This tension is alleviated considering theuncertainty on predicting the count rate in the ROSAT detector, and on thesystematics affecting the magnetic field determination. Overall we find goodagreement between the CAB parameters for the four clusters studied so far.

A Cosmic ALP Background and the Cluster Soft X-ray Excess in A665, A2199 and A2255