Non-thermal intracluster medium: a simultaneous interpretation of the central soft X-ray excess and WMAP’s detection of reduced Sunyaev-Zel’dovich Effect

WMAP’s detection of the Sunyaev-Zel’dovich effect (SZE) at a much reduced level among several large samples of rich clusters is interpreted in terms of conventional physics. It is by now widely believed that the central soft X-ray and EUV excess found in some clusters cannot be of thermal origin, due to problems with rapid gas cooling and the persistent non-detection of the O VII line, but may arise from inverse-Compton scattering between intracluster relativistic electrons and the cosmic microwave background (CMB). In fact, recently Chandra and XMM observations of the soft X-rays from Abell 3112 are well fitted by the sum of a power law and a thermal virialized gas component of comparable luminosities. Therefore the missing SZE flux could simply be due to an overestimate of the central density of virialized electrons which scatter the CMB. We also considered if higher energy electrons drawn from the same power-law population as those responsible for the soft excess may synchrotron radiate in the intracluster magnetic field of strength B . a few μG to produce cluster microwave emissions in the WMAP passbands that account for the missing SZE flux. Either explanation of the WMAP anomaly would bolster the current model of central cluster soft excesses, viz. non-thermal activities prevail in the core of at least some clusters. The energetic electrons may originate from AGN jet injection, then distributed cluster-wide with accompanying in situ Fermi acceleration, by Alfven waves. However the missing thermal electron explanation provides a less demanding model.