The Contribution of the Obscuring Torus to the X--Ray Spectrum of Seyfert Galaxies: A Test for the Unification Model

Abstract

The presence of an obscuring torus around the nucleus of Seyfert galaxies, as supposed in the popular unification scheme, can strongly modify the X--ray spectrum for both type 1 and 2 Seyfert galaxies. We find that if the torus is Compton thick, it can scatter at small angles a significant fraction of the nuclear radiation, and contribute to the continuum of Seyfert 1 galaxies above $\sim$10 keV, and to the fluorescence iron line at 6.4 keV. At large inclination angles and for large torus column densities, the spectrum is attenuated by photoabsorption and Compton scattering, while the iron fluorescence line produced by the torus can have large equivalent widths. Even after dilution by the continuum scattered by the warm material outside the torus, this iron line could be strong enough to explain the ``cold" component in the spectrum of NGC 1068. We stress that the complex pattern of the predicted variability can be a powerful tool for constraining the parameters of the model, such as the column density of the torus, its inclination and the amount of warm scattering material.