Open AccessOn the origin of soft X‐rays in obscured AGN: answers from high‐resolution spectroscopy with XMM–Newton
Author(s) -
Guainazzi Matteo,
Bianchi Stefano
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.11229.x
Subject(s) - physics , photoionization , active galactic nucleus , astrophysics , ionization , spectroscopy , luminosity , galaxy , radiative transfer , spectral line , spectrometer , astronomy , ion , optics , quantum mechanics
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 Heα triplet (once normalized to the O viii Lyα) coupled with the integrated luminosity in He‐ and H‐like oxygen lines.