High-energy X-ray spectra of Seyferts and Unification schemes for active galactic nuclei

Abstract

The Unified Model of active galactic nuclei (AGN) predicts that the sole difference between type 1 and 2 Seyfert galaxies nuclei is the viewing angle with respect to an obscuring structure around the nucleus. High-energy photons above 20 keV are not affected by this absorption if the column is Compton thin, so their 30-100 keV spectra should be the same. However, the observed spectra at high energies appear to show a systematic difference, with type 1 Seyfert galaxies having Γ ~ 2.1 whereas type 2 Seyfert galaxies are harder with Γ ~ 1.9. We estimate the mass and the accretion rate of Seyferts detected in these high-energy samples, and show that they span a wide range in L/L_Edd. Both black hole binary systems and AGN show a correlation between spectral softness and Eddington fraction, so these samples are probably heterogeneous, spanning a range of intrinsic spectral indices which are hidden in individual objects by poor signal-to-noise ratio. However, the mean Eddington fraction for the type 1 Seyfert galaxies is higher than for the type 2 Seyfert galaxies, so the samples are consistent with this being the origin of the softer spectra seen in type 1 Seyfert galaxies. We stress that high-energy spectra alone are not necessarily a clean test of Unification schemes, but that the intrinsic nuclear properties should also change with L/L_Edd.