The role of absorption and reflection in the X-ray spectrum of active galactic nuclei

In the 2-10 keV range, the AGN continuum is generally well represented by a single power law but at lower energies it displays an excess with respect to the extrapolation of this power law, called the "soft X-ray excess"; the nature of this component is still under discussion. Until now the soft X-ray excess was attributed either to the reflection of the hard X-rays on the accretion disk, or to the presence of an additional Comptonizing medium. This feature could also be due to the absorption of an intrinsically steep power law source (whose origin is not clear) by a medium with a very large dispersion velocity (as a relativistic wind). Understanding the nature of the soft X-ray excess is essential for our knowledge of the Warm Absorber, the primary spectrum, and the accretion flow process. We have therefore examined the pros and cons of the reflection and absorption models. The observed soft X-ray spectra may probably be modeled by an "hybrid" model: absorption and reflection.