2006A&A...449..493C

The 2-10keV continuum of AGN is well represented by a single power law, generally attributed to a hot Comptonizing medium, such as a corona above the accretion disk. At lower energies the continuum displays an excess with respect to the extrapolation of this power law, called the ``soft X-ray excess''. Until now it was attributed either to reflection of the hard X-ray source by the accretion disk or to the presence of an additional Comptonizing medium. An alternative solution is that a single power law correctly represents both the soft and the hard X-ray emission, and the soft X-ray excess is an artefact due to the absorption of the primary power law by a relativistic wind. We examine the advantages and drawbacks of the reflection versus absorption models. We argue that in the absorption hypothesis, the absorbing medium should be in total pressure equilibrium to constrain the spectral distribution which otherwise would be too strongly variable in time and from one object to the other, as compared to observations. We conclude that some X-ray spectra, in particular those with strong soft X-ray excesses, can be modelled by absorption in the 0.3-10keV range. However, due to the lack of a complete grid of models and good data extending above 10keV, we are not able to conclude that all objects can be accommodated by such models. These absorption models imply either strong relativistic outflowing winds with mass rates of the order of the Eddington value (or even larger), or quasi-spherical inhomogeneous accretion flows. Only weak excesses can be modelled by reflection, unless the primary continuum is not directly seen. A reflection model absorbed by a modest relativistic wind could be the best solution to the problem.