On the origin of soft X-rays in obscured AGN:: answers from high-resolution spectroscopy with XMM-Newton

We present results of a high-resolution soft X-ray (0.2-2 keV) spectroscopic study of a sample of 69 nearby obscured active galactic nuclei (AGNs) observed with the reflection grating spectrometer (RGS) onboard XMM-Newton. This is the largest sample ever studied with this technique so far. The main conclusions of our study can be summarized as follows: (i) narrow radiative recombination continua are detected in about 36 per cent of the objects in our sample (in 26 per cent their intrinsic width is <= 10 eV); (ii) higher order transitions are generally enhanced with respect to pure photoionization, indicating that resonant scattering plays an important role in the ionization/excitation balance. These results support the scenario, whereby the active nucleus is responsible for the X-ray 'soft excess' almost ubiquitously observed in nearby obscured AGNs via photoionization of circumnuclear gas. They confirm on a statistical basis the conclusions drawn from the detailed study of the brightest spectra in the sample. Furthermore, we propose a criterion to statistically discriminate between AGN-photoionized sources and starburst galaxies, based on intensity of the forbidden component of the O VII He alpha triplet (once normalized to the O VIII Ly alpha) coupled with the integrated luminosity in He- and H-like oxygen lines.