2010A&A...516A...9D

The physical conditions of the gas forming the narrow-line regions (NLR) in active galactic nuclei (AGN) have been extensively studied in the optical band. Recently detailed X-ray studies have shown how the emission in the 0.1-2keV band detected in Seyfert 2 galaxies is associated to gas lying close to or associated with the NLR. We take advantage of the spectacular extension (∼15'') of the NLR in the type II Seyfert galaxy NGC 5252 and of the complementary characteristics of XMM-Newton and Chandra to investigate the physical conditions of the gas in this galaxy. The X-ray data from XMM-Newton are used to define the spectral properties of the ionising nuclear source. The Chandra data are used to trace the spatial characteristics of the soft X-ray emission. This information is then compared to the optical HST characteristics of the NLR in NGC 5252. The X-ray spectrum of the nucleus of NGC 5252 is intrinsically flat (Γ∼1.4-1.5) and absorbed by neutral gas with a column density N_H∼10²²cm^–2. Below ∼1keV a soft excess is detected. The high-resolution spectrum obtained with the XMM-Newton RGS shows emission lines in the 0.2-1.5keV range which strongly indicate that the soft X-ray component is essentially due to ionised gas. Moreover, the soft X-ray emission is spatially resolved around the nucleus and well overlaps the images obtained in narrow optical bands centred around the [Oiii] emission line at 5007 Å. The [Oiii]/soft-X flux ratios along the ionisation cones are basically constant. This indicates that the electron density does not significantly deviate from the r^–2 law (constant ionisation parameter) moving outward from the nucleus. This result combined with previous optical studies suggests two plausible but different scenarios in the reconstruction of the last ∼30000 years of history of the central AGN. The most promising one is that the source is indeed a ``quasar relic'' with a steady and inefficient energy release from the accretion of matter onto the central super-massive black-hole. This scenario is suggested also by the flat nuclear X-ray spectrum that evokes an advection dominated accretion flow (ADAF) like emission mechanism.