The Origin of the Extragalactic X-Ray Background

  • G. Setti
Conference paper
Part of the Astrophysics and Space Science Library book series (ASSL, volume 164)

Abstract

Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, it is well known that 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. 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, prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs 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.

Keywords

Cosmic Microwave Background Active Galactic Nucleus Radio Galaxy Cosmological Evolution Surface Brightness Fluctuation 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Kluwer Academic Publishers 1990

Authors and Affiliations

  • G. Setti
    • 1
  1. 1.European Southern ObservatoryGarching bei MünchenFederal Republic of Germany

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