X-ray spectral signatures of very thick cold matter in the Spectra of seyfert galaxies

Summary Over the last few years, GINGA observations of several bright Seyfert galaxies have revealed the existence of a new flat component in the X-ray spectrum of these objects above 8 keV. This component (the ?high-energy bump?) is probably produced by the reprocessing of the intrinsic emission (through Compton scattering and photoelectric absorption) by a very thick and cold medium in the environment of the central source, such as an accretion disk. An alternative explanation is provided by partial covering and reprocessing by thick clouds, whose existence is supported by the latest results on the broad line region obtained by line reverberation measurements in the optical-ultraviolet band. The presence of iron fluorescent lines at 6.4 keV with an equivalent width ≅ (100÷200) eV in all those galaxies supports both scenarios. The implications of the presence of the high-energy bump on the spectral shape of the intrinsic continuum, the level of the soft X-ray excess and the X-ray background are briefly pointed out. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.