Seyfert galaxies in the local Universe (z ≤ 0.1): the average X-ray spectrum as seen by BeppoSAX

¹ INAF/IASF-Bo, via Gobetti 101, 40129 Bologna, Italy
² Dipartimento di Astronomia dell'Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: mauro.dadina@iasfbo.inaf.it

Received: 29 March 2007
Accepted: 6 February 2008

Abstract

The BeppoSAX archive is currently the largest reservoir of high sensitivity simultaneous soft and hard-X ray data of Seyfert galaxies. From this database all the Seyfert galaxies (105 objects of which 43 are type I and 62 are type II) with redshift lower than 0.1 have been selected and analyzed in a homogeneous way. Taking advantage of the broad-band coverage of the BeppoSAX MECS and PDS instruments (~2-100 keV), the X-ray data so collected allow us to infer the average spectral properties of nearby Seyfert galaxies included in the original sample and, most notably the photon index (Γ ~ 1.8), the high-energy cut-off (E_c ~ 290 keV), and the relative amount of reflection (R ~ 1.0). The data collected have been used to test some basic assumptions of the unified scheme for active galactic nuclei. The distributions of the isotropic indicators used here (photon index, relative amount of reflection, high-energy cut-off and narrow FeKα energy centroid) are similar in type I and type II objects while the absorbing column and the iron line equivalent width significantly differ between the two classes of active galactic nuclei with type II objects displaying larger columns (N_H ~ 3.710²² and 6.110²³ cm^-2 for type I and II objects respectively) and equivalent width (EW ~ 220 and 690 eV for type I and II sources respectively). Confirming previous results, the narrow FeKα line is consistent, in Seyfert 2, with being produced in the same matter responsible for the observed obscuration. These results support the basic picture of the unified model. Moreover, the presence of a X-ray Baldwin effect in Seyfert 1 has been measured using the 20-100 keV luminosity . Finally, the possible presence of a correlation between the photon index and the amount of reflection is confirmed thus indicating thermal Comptonization as the most likely origin of the high energy emission for the active galactic nuclei included in the original sample.