The Average X-Ray/Gamma-Ray Spectra of Seyfert Galaxies from GINGA and OSSE and the Origin of the Cosmic X-Ray Background

We have obtained the first average 2-500 keV spectra of Seyfert galaxies, using the data from Ginga and Compton Gamma-Ray Observatory's OSSE. Our sample contains three classes of objects with markedly different spectra: radio-quiet Seyfert 1's and 2's, and radio-loud Seyfert 1's. The average radio-quiet Seyfert 1 spectrum is well-fitted by a power law continuum with the energy spectral index α ~ 0.9, a Compton reflection component corresponding to α ~ 2π covering solid angle, and ionized absorption. There is a high-energy cutoff in the incident power law continuum: the e-folding energy is E_c_~0.6_- 0.3_^+0.8^ MeV. The simplest model that describes this spectrum is Comptonization in a relativistic optically-thin thermal corona above the surface of an accretion disk. Radio-quiet Seyfert 2's show strong neutral absorption, and there is an indication that their X-ray power laws are intrinsically harder, although the Seyfert 1 spectrum with α = 0.9 and strong reflection cannot be ruled out by the data. Finally, the radio-loud Seyfert spectrum has α ~ 0.7, moderate neutral absorption, E_c_ = 0.4_-0.2_^+0.7^ MeV, and no or little Compton reflection. This is incompatible with the radio-quiet Seyfert 1 spectrum, and probably indicating that the X-rays are beamed away from the accretion disk in these objects. The average spectra of Seyferts integrated over redshift with a power- law evolution can explain the hard X-ray spectrum of the cosmic background. The hump at ~30 keV in that spectrum is due to the dominant contribution of Seyfert 2's.