The properties of the circumnuclear regions in the Circinus galaxy

¹ Dipartimento di Fisica, Università degli studi Roma Tre, via della Vasca Navale 84, 0046 Roma, Italy e-mail: massaro@roma2.infn.it
² Dipartimento di Fisica, Università degli studi di Tor Vergata, via della Ricerca scientifica 1, 00133 Roma, Italy
³ XMM-Newton Science Operations Center, European Space Astronomy Center, ESA, Apartado 50727, 28080 Madrid, Spain
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138

Received: 24 December 2005
Accepted: 23 March 2006

Abstract

We analysed the spectra from the XMM-Newton and the three longest Chandra observations of the Circinus Galaxy, the X-ray brightest, Compton-thick Seyfert 2 galaxy, to derive the physical and morphological parameters of the circumnuclear matter. We compare the line and continuum parameters with extensive calculations performed with the well-known photoionisation code CLOUDY. The line spectrum cannot be explained by a single zone, as three different reflecting regions are required. The first has low ionisation, accounting for the prominent iron Kα line, and is likely to be identified with the “torus” envisaged in unification models for Seyfert galaxies; a lower limit of 0.05 pc to the distance of this matter from the nucleus has been estimated, using new constraints on the ionisation state of iron (≤FeX). The second zone is moderately ionised and optically thin ( 0.2), and it accounts for the oxygen to silicon lines. Finally, a third, highly ionised and optically thin reflector is required for the detection of the He-like iron line.