Observational constraints on models for the cluster soft excess emission

Although the cluster soft excess phenomenon is confirmed by XMM-Newton observations of many clusters. the cause of this new component of radiation remains an enigma. The two mechanisms proposed in the late 90's, viz. thermal emission from a massive warm baryonic gas and inverse-Compton scattering between cosmic rays and the microwave background, are still to date the only viable interpretations of the soft excess. In as much as cosmic rays cannot exist at the vast void of a cluster's outskirts, warm gas also cannot be present with any significant degree of abundance at the center of a cluster. In this sense, one could say that both models have their merits. and account for the soft excess in different spatial regions. There is however no clincher evidence that points definitively to the correctness of either explanation. Thus, the door remains open for more exotic scenarios that could even consider the detected emission as signature of some dark matter process. In fact. the absence of absorption lines in the spectrum of background quasars along sightlines going through the outer radii of clusters argues against the thermal model within the domain where it most suitably applies. On the other hand, if the central excesses are due to cosmic rays, the pressure of the proton component will be large enough to choke a cooling flow, and missions like GLAST may have the sensitivity to detect the gamma rays that ensue from proton-gas interactions. However the diagnosis may turn out to be. it is likely that the new radiation represents something of cosmological importance.