ALP Conversion and the Soft X-ray Excess in the Outskirts of the Coma
  Cluster
  It was recently found that the soft X-ray excess in the center of the Coma
cluster can be fitted by conversion of axion-like-particles (ALPs) of a cosmic
axion background (CAB) to photons. We extend this analysis to the outskirts of
Coma, including regions up to 5 Mpc from the center of the cluster. We extract
the excess soft X-ray flux from ROSAT All-Sky Survey data and compare it to the
expected flux from ALP to photon conversion of a CAB. The soft X-ray excess
both in the center and the outskirts of Coma can be simultaneously fitted by
ALP to photon conversion of a CAB. Given the uncertainties of the cluster
magnetic field in the outskirts we constrain the parameter space of the CAB. In
particular, an upper limit on the CAB mean energy and a range of allowed
ALP-photon couplings are derived.
