ALP Conversion and the Soft X-ray Excess in the Outskirts of the Coma Cluster

TL;DR

This paper investigates whether axion-like particles converting into photons can explain the soft X-ray excess observed in both the center and outskirts of the Coma cluster, using ROSAT data.

Contribution

It extends previous ALP conversion analysis to the outskirts of Coma, constraining CAB parameters based on X-ray data and magnetic field uncertainties.

Findings

Soft X-ray excess in Coma can be fitted by ALP to photon conversion.

Constraints on CAB mean energy and ALP-photon coupling are derived.

The model explains X-ray excess both at the center and outskirts.

Abstract

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.