While in most cases the origin of the soft excess emission is difficult to prove unambiguously, due to problems with instrument calibration and unknown background/foreground emission level, the independent detection of the phenomenon with many instruments gives confidence in the genuine nature of the phenomenon in a number of clusters. Both thermal and non-thermal emission mechanisms are probably at work in producing the soft excess emission in clusters.
Acknowledgements The authors thank ISSI (Bern) for support of the team "Non-virialized X-ray components in clusters of galaxies". F.D. acknowledges support from CNES. SRON is supported financially by NWO, the Netherlands Foundation for Scientific Research. J.N. acknowledges support from the Academy of Finland. We thank R. Lieu, M. Bonamente and A. Fabian for pointing out the possible modifications of the soft excess emission when using more recent determinations of the hydrogen absorption along the line of sight (see note added in proof).