Reflection in Seyfert galaxies and the unified model

Abstract

We present a study of the average hard X-ray spectra of Seyfert galaxies accumulating 100 Ms of INTEGRAL observing time. Our aim is to test the unified model of Active Galactic Nuclei, and to constrain differences and similarities between different classes of objects. We analysed all public INTEGRAL IBIS/ISGRI data available on 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1, 29 Seyfert 1.5, 78 Seyfert 2, and 14 Narrow Line Seyfert 1. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17-250 keV energy range. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model. The average spectrum of midly obscured Seyfert 2s is different, featuring a broad excess of emission at 30 keV that we interpret as a strong reflection component. This correlation between reflection and absorption suggests that the Seyfert type is determined by the environment rather than by inclination. Furthermore this large reflection component might reduce the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. A simple fit to the CXB shows that our findings might imply that the fraction of Compton-thick sources is not larger than 10%.