Energy-dependent variability and the origin of the soft X-ray excess in active galactic nuclei

The origin of the soft excess seen in many active galactic nuclei (AGN) below similar to 1 keV is still an unsolved problem. It is unlikely to represent a true continuum component as its characteristic 'temperature' shows a remarkable constancy over a wide range of AGN luminosity and black hole mass. This instead favours an association with atomic processes, in particular with the increase in opacity between 0.7 and 2 keV associated with partially ionized O and Fe. The opacity jump can give rise to a soft excess through either reflection or transmission, and both scenarios can fit the spectra equally well as long as there is strong velocity shear to smear out the characteristic narrow atomic features. Here we use orthogonal constraints from the energy-dependent variability. The rms spectra seen in XMM-Newton AGN data often show a broad peak between 0.7 and 2 keV. We show that the absorption model can explain the data well if the ionization state of the smeared absorption responds to luminosity changes in the continuum.