An absorption origin for the soft excess in Seyfert 1 AGN

The soft excess seen in the X–ray spectra of many AGN can be well modelled by reflection from a partially ionised accretion disc. However, this often requires extreme parameters, both in terms of the underlying spacetime and the reflection geometry. An alternative model uses similarly partially ionised, velocity smeared material but from an accretion disc wind seen in absorption. We explicitly compare these two models for the origin of the soft excess using XMM-Newton data for 30 low redshift PG (Palomar-Green) QSOs, comprising 9 Narrow Line Seyfert 1’s and 13 Broad Line Seyfert 1’s together with an additional 12 Narrow Line Seyfert 1’s. We find that while reflection and absorption give comparably good fits to the data, so cannot be distinguished statistically, the absorption model allows a much clearer correspondence with the stellar mass black holes. All the objects are high mass accretion rate AGN, so should be analogous to the high/soft and very high states in black hole binaries. The intrinsic spectral indices derived from the absorption model are all consistent with a one–to–one mapping between spectral state and AGN type, with the NLS1’s having softer spectra corresponding to the very high state, while the broad line AGN have Γ = 2−2.2 as expected for the high/soft state. However, we also use the simultaneous OM data to derive the ratio of disc to total accretion power which is another tracer of spectral state in X–ray binaries. This does not always show that the disc in NLS1’s contributes less than 80 per cent of the total power, as expected for a very high state. We suggest that this is an artifact of the standard disc models used to determine the disc luminosity in our fits. The disc seen in the very high state of black hole binaries is often observed to be distorted from the standard shape, and a similar effect in NLS1’s could recover the one–to–one mapping between black hole binary spectral state and AGN type, reinforcing a simple correspondence between stellar and supermassive black holes.