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 a Seyfert galaxy, as supposed in the popular unification scheme, can strongly modify the X-ray spectrum for both type 1 and type 2 Seyfert galaxies. We study this problem by means of Monte Carlo simulations, finding 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 ∼ 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. Identifying the scattering medium with the warm absorber seen in Seyfert 1 galaxies, we derive a lower limit of the inclination angle in this source of about 50°. Furthermore, an energy-budget argument suggests a value of about 55°-70°. 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. Observations of the 6.4-keV fluorescence iron line with good energy resolution can indicate the relative importance of the torus line emission with respect to cold material in the vicinity of the nuclear source, such as an accretion disc.