  The study of soft X-ray emission of 38 X-ray selected galaxy clusters
observed by ROSAT PSPC indicates that the soft excess phenomenon may be a
common occurrence in galaxy clusters. Excess soft X-ray radiation, above the
contribution from the hot intra-cluster medium, is evident in a large fraction
of sources, and is clearly detected with large statistical significance in the
deepest observations. The investigation relies on new, high resolution 21 cm HI
observations. The sample selection also features analysis of infra-red images,
to further ensure reliability of results with respect to the characteristics of
Galactic absorption. The possibility of background or calibration effects as
cause of the excess emission is likewise investigated; a detailed analysis of
the distribution of the excess emission with respect to detector position and
Galactic HI column density shows that the excess emission is a genuine
celestial phenomenon. We find evidence for a preferential distribution of the
soft excess emission at distances larger than ~150-200 kpc from the centers of
clusters; this behavior may be naturally explained in the context of a
non-thermal Inverse-Compton scenario. Alternatively, we propose that the
phenomenon may be caused by thermal emission of very large-scale `warm'
filaments seen in recent hydrodynamic simulations. This new interpretation
relieves the very demanding requirements of either the traditional
intra-cluster `warm' gas and the non-thermal scenarios. We also investigate the
possibility of the soft excess originating from unresolved, X-ray faint cluster
galaxies.
