SIMBAD references

The study of soft X-ray emission of 38 X-ray-selected galaxy clusters observed by the ROSAT Position Sensitive Proportional Counter 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 intracluster 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 H I observations. The sample selection also features analysis of infrared 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 H I 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 nonthermal 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 intracluster ``warm'' gas and the nonthermal scenarios. We also investigate the possibility of the soft excess originating from unresolved, X-ray-faint cluster galaxies.