The Average X-Ray/Gamma-Ray Spectra of Seyfert Galaxies From Ginga and OSSE and the Origin of the Cosmic X-Ray Background
NAVAL RESEARCH LAB WASHINGTON DC E O HULBURT CENTER FOR SPACE RESEARCH
Pagination or Media Count:
We have obtained the first average 2-500 keV spectra of Seyfert galaxies, using the data from Ginga and GRO OSSE. Our sample contains 3 classes of objects with markedly different spectra radio-quiet Seyfert 1s and 2s, and radio-loud Seyfert 1s. The average radio-quiet Seyfert 1 spectrum is well- fitted by a power law continuum with the energy spectral index approximately equal 0.9, a Compton reflection component corresponding to a 2x covering solid angle, and ionized absorption. There is a high-energy cutoff in the incident power law continuum the e-folding energy is Esub c approximately equal 0.6 sup 0.8, sub -0.3MeV. 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 2s show strong neutral absorption, and there is an indication that their X-ray power laws are intrinsically harder, although the Seyfert 1 spectrum with 0.9 and strong reflection cannot be ruled out by the data. Finally, the radio-loud Seyfert spectrum has alpha approximately equal 0.7, moderate neutral absorption, E sub c 0.4sup 0.7, sub -0.2 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 2s.