On the origin of X-ray oxygen emission lines in obscured AGN

Abstract

We present the Catalogue of High Resolution Spectra of Obscured Sources (CHRESOS) from the XMM-Newton Science Archive. It comprises the emission-line luminosities of H- and He-like transitions from C to Si, and the Fe 3C and Fe 3G L-shell ones. Here, we concentrate on the soft X-ray O VII (f) and O VIII Lyα emission lines to shed light on to the physical processes with which their formation can be related to active galactic nucleus (AGN) versus star-forming regions. We compare their luminosity with that of two other important oxygen key lines [O III] λ5007 Å, in the optical, and [O IV] 25.89 μm, in the infrared (IR). We also test O VII (f) and O VIII Lyα luminosities against that of continuum bands in the IR and hard X-rays, which point to different ionization processes. We probe into those processes by analysing photoionization and collisional ionization model predictions upon our lines. We show that both scenarios can explain the formation and observed intensities of O VII (f) and O VIII Lyα. By analysing the relationships between O VII (f) and O VIII Lyα, and all other observables: [O III] λ5007 Å, [O IV] 25.89 μm emission lines, and mid-infrared (MIR) 12 μm, far-infrared (FIR) 60 and 100 μm, 2-10 and 14-195 keV continuum bands, we conclude that the AGN radiation field is mainly responsible of the soft X-ray oxygen excitation.