X-ray imaging of the ionisation cones in NGC 5252

¹ INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: dadina@iasfbo.inaf.it
² Università degli Studi di Bologna, Dip. di Astronomia, via Ranzani 1, 40127 Bologna, Italy
³ European Space Astronomy Center of ESA, Apartado 50727, 28080 Madrid, Spain
⁴ Dipartimento di Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00046 Roma, Italy
⁵ INAF/Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 24 November 2009
Accepted: 6 March 2010

Abstract

Context. 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 0.1–2 keV band detected in Seyfert 2 galaxies is associated to gas lying close to or associated with the NLR.

Aims. 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.

Methods. 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.

Results. 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~10²² cm^-2. Below ~1 keV a soft excess is detected. The high-resolution spectrum obtained with the XMM-Newton RGS shows emission lines in the 0.2–1.5 keV 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.

Conclusions. This result combined with previous optical studies suggests two plausible but different scenarios in the reconstruction of the last ~30 000 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.