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The Extragalactic X-Ray and γ-ray Backgrounds

Published online by Cambridge University Press:  08 February 2017

G. Setti
G. Setti*
Affiliation:
European Southern Observatory Karl-Schwarzschild-Str. 2 D-0846 Garching bei München FRG

Abstract

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The long-standing problem of the origin of the extragalactic X-ray background (XRB) is reviewed. Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, the interpretation in terms of a hot intergalactic gas (IGG) requires a rather extreme energy supply and a gas density conflicting with the baryon density upper limit derived from primordial nucleosynthesis calculations in the standard hot big-bang model. A summary discussion of the estimated contributions from the integrated X-ray emission of known classes of extragalactic discrete sources at a reference energy of 2 keV is given. Although these estimates are still uncertain, the subtraction of a “minimum” contribution drastically modifies the 40 keV thermal shape, which is the prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs (Seyfert type 1 nuclei) at redshift z = 1 − 2 may in fact saturate the 2 keV XRB, but their observed hard X-ray spectra are on the average unlike (much too steep) that of the XRB. This has led a number of authors to postulate new classes of sources and some exotic models which are briefly summarized. However, if a recently proposed unified scheme of AGNs holds, then the bulk of the XRB intensity can be explained independently of the observed spectral differences and with a mild cosmological evolution. The origin of the extragalactic γ-ray background is briefly commented upon in the concluding remarks.

Type
IV. Extragalactic Background Radiation and Cosmology
Information
Symposium - International Astronomical Union , Volume 139: The Galactic and Extragalactic Background Radiation , 1990 , pp. 345 - 356
Copyright
Copyright © Kluwer 1990 

References

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