The origin of the soft excess seen in many AGN below \sim 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 0.7–2 keV associated with partially ionized O and Fe .
The opacity jump can give rise to a soft excess either through 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 0.7–2 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 .