Reflection in Seyfert galaxies and the unified model of AGN^⋆

¹ ISDC Data Centre for Astrophysics, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: claudio.ricci@unige.ch
² Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland

Received: 25 December 2010
Accepted: 15 April 2011

Abstract

We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We aim to test the unified model of active galactic nuclei, and constrain differences and similarities between different classes of objects. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1s, 29 Seyfert 1.5s, 78 Seyfert 2s, and 14 narrow-line Seyfert 1s. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17–250 keV energy range. We performed a detailed spectral analysis using both a model-independent and a model-dependent approach. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cutoff energy of E_C ≳ 200 keV, and a photon index of Γ ≃ 1.8. The average optical depth of the Comptonizing medium is consistent for the different classes (τ ≃ 0.8). Compton-thin Seyfert 2s show a reflection component stronger than Seyfert 1s and Seyfert 1.5s. Most of this reflection is due to mildly obscured (10²³   cm^-2 ≤ N _H < 10²⁴   cm^-2) Seyfert 2s, which have a significantly stronger reflection component ($\mathit{R}\mathrm{=}\mathrm{2.}{\mathrm{2}}_{-1.1}^{\mathrm{+}\mathrm{4.5}}$) than Seyfert 1s (R ≤ 0.4), Seyfert 1.5s (R ≤ 0.4), and lightly obscured (N _H < 10²³   cm^-2) Seyfert 2s (R ≤ 0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2s might cover a large fraction of the X-ray source, and contain clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2s reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being  ~10%. The spectra of Seyfert 2s with and without polarized broad lines do not show significant differences, the only difference between the two samples being the higher hard X-ray and bolometric luminosity of Seyfert 2s with polarized broad lines. The average hard X-ray spectrum of narrow-line Seyfert 1s is steeper than those of Seyfert 1s and Seyfert 1.5s, probably due to a lower energy of the cutoff.