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

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 (D < ∼ 22 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 10 38 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 10 20 and 10 25 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.

X-ray spectral parameters for a sample of 95 active galactic nuclei

“…Therefore, the samples for different Seyfert types have reliable statistical differences of absorption. Our results are in accord with the results of Cappi et al 2006;Singh et al 2011;Winter et al 2012; they correspond to the predicted trends of increasing of FeK EW within UM.…”

“…The last three observational parameters depend on the state of the accretion disk and of the presence of a gas-dusty torus on the line of sight (Awaki 1991;Cappi et al 2006;Petrucci et al 2001;Ueda et al 2007). The torus optical thickness and its geometry shape reveal themselves in the anisotropy of the absorption and scattering (Ricci et al 2011;Ueda et al 2007).…”

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

“…Nevertheless, we cannot rule it out based upon the present evidence, and we also note that the presence of a PL scatt component, itself, is not surprising, with the measured f scatt value of a few per cent being fully consistent with that seen in other nearby AGN (e.g. Cappi et al 2006). We have introduced this only as one possible scenario, and as we will discuss in the following section, more complex models could alleviate the need for this screen altogether.…”

The dependence of X-ray AGN activity on host galaxy properties and environment

“…Specifically, at z < ∼ 0.1 the normalisation of the AGN luminosity function is different by ∼0.5 dex when using emission-lines and X-ray selection criteria ). In the framework of the unified scheme of AGN these differences were often suggested to be due to the existence of an AGN population which is heavily obscured in the X-ray regime (Levenson et al 2002;Cappi et al 2006). Risaliti et al (1999) reports that 75% of optically selected Seyfert-2 galaxies are heavily obscured (n H > 10 23 cm −2 ), and about half are Compton thick (n H > 10 24 cm −2 ); as shown in Fig.…”