2008A&A...482..499D

The nature and origin of the soft X-ray excess in radio-quiet AGN is still an open issue. The interpretation in terms of thermal disc emission has been challanged by the discovery of the constancy of the effective temperature despite the wide range of Black Hole masses of the observed sources. Alternative models are reflection from ionized matter and absorption in a relativistically smeared wind. We analyzed XMM-Newton observations of four luminous radio-quiet AGN with the aim of characterising their main properties and in particular the soft excess. Different spectral models for the soft excess were tried: thermal disc emission, Comptonization, ionized reflection, relativistically smeared winds. Comptonization of thermal emission and smeared winds provide the best fits, but the other models also provide acceptable fits. All models, however, return parameters very similar from source to source, despite the large differences in luminosities, Black Hole masses and Eddington ratios. Moreover, the smeared wind model requires very large smearing velocities. The UV to X-ray fluxes ratios are very different, but do not correlate with any other parameter. No fully satisfactory explanation for the soft X-ray excess is found. More informative data, such as observations in a broader energy band, are needed to make further progress.