X-ray spectral survey with XMM–Newton of a complete sample of nearby Seyfert galaxies

¹ INAF-IASF Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: cappi@bo.iasf.cnr.it
² Instituto de Fisica de Cantabria (CSIC-UC), Avda de los Castros, 39005 Santander, Spain
³ INAF- Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF- Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁵ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁶ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁷ Dipartimento di Astronomia, Universita' degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁸ XMM–Newton Science Operation Center/RSSD-ESA, Apartado 50727, 28080 Madrid, Spain
⁹ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ Laboratory for High Energy Astrophysics, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 23 July 2005
Accepted: 28 September 2005

Abstract

Results obtained from an X-ray spectral survey of nearby Seyfert galaxies using XMM–Newton are reported. The sample was optically selected, well defined, complete in B magnitude, and distance limited: it consists of the nearest ( Mpc) 27 Seyfert galaxies (9 of type 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315) sample. This is one of the largest atlases of hard X-ray spectra of low-luminosity active galaxies ever assembled. All nuclear sources except two Seyfert 2s are detected between 2 and 10 keV, half for the first time ever, and average spectra are obtained for all of them. Nuclear luminosities reach values down to 10³⁸ erg s^-1. The shape of the distribution of X-ray parameters is affected by the presence of Compton-thick objects (30% among type 2s). The latter have been identified either directly from their intense FeK line and flat X-ray spectra, or indirectly with flux diagnostic diagrams which use isotropic indicators. After taking into account these highly absorbed sources, we find that (i) the intrinsic X-ray spectral properties (i.e., spectral shapes and luminosities above 2 keV) are consistent between type 1 and type 2 Seyferts, as expected from “unified models”; (ii) Seyfert galaxies as a whole are distributed fairly continuously over the entire range of N_H, between 10²⁰ and 10²⁵ cm^-2; and (iii) while Seyfert 1s tend to have lower N_H and Seyfert 2s tend to have the highest, we find 30% and 10% exceptions, respectively. Overall the sample is of sufficient quality to well represent the average intrinsic X-ray spectral properties of nearby active galactic nuclei, including a proper estimate of the distribution of their absorbing columns. Finally, we conclude that, with the exception of a few cases, the present study agrees with predictions of unified models of Seyfert galaxies, and extends their validity down to very low luminosities.