Emission ratio between different types of Active Galactic Nuclei in the hard X-ray regime
This graph shows the ratio of the emission between two different classes of Active Galactic Nuclei (AGN) at hard X-ray energies as observed with ESA's INTEGRAL observatory.
The two AGN classes considered here are Mildly Obscured (MOB) Seyfert 2 galaxies and Seyfert 1 galaxies, which are characterised by stronger and weaker absorption, respectively, at lower energies (i.e. from the infrared to the soft X-ray bands).
The graph shows that MOB Seyfert 2 galaxies exhibit a strong excess of emission in the energy range 20-60 keV when compared to Seyfert 1 galaxies. The excess emission is a signature of X-rays being reflected off neutral hydrogen gas in the dense clouds that surround the black hole and accretion disc, possibly the same clouds responsible for absorption at lower energies.
The unified model of AGN predicts that these objects should have the same behaviour in the hard X-ray band, meaning that the ratio is expected to be equal to 1. The observed discrepancy raises questions about some aspects of this model; in particular, the result indicates that the presumed existence, in all AGN, of an anisotropic, toroidal structure of absorbing gas and dust needs revision.