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The Sources of the Hard X-Ray Background

Published online by Cambridge University Press:  25 May 2016

Giancarlo Setti  and
Andrea Comastri
Giancarlo Setti
Affiliation:
Dipartimento di Astronomia, Università di Bologna, Osservatorio Astronomico di Bologna, Istituto di Radioastronomia del CNR, Via P. Gobetti, 101 - 40129 Bologna - Italy
Andrea Comastri
Affiliation:
Dipartimento di Astronomia, Università di Bologna, Osservatorio Astronomico di Bologna, Istituto di Radioastronomia del CNR, Via P. Gobetti, 101 - 40129 Bologna - Italy

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The hard component (3 keV – ~ MeV) of the X-ray background (XRB) comprises the largest portion, ~ 90%, of the overall XRB intensity. The observed isotropy (the entire Galaxy is transparent above 3 keV) provides a prima facie evidence of its prevailing extragalactic nature. A large fraction (~ 75%) of the energy flux falls in the 3 – 100 keV band, the corresponding energy density being ≃ 5×10−5 eV cm−3, of which 50% is confined to the narrower 3 – 20 keV band. Although the energy flux carried by the XRB is relatively small compared to other extragalactic backgrounds, it was soon realized that it cannot be accounted for in terms of sources and processes confined to the present epoch. An analysis of the combined observed spectra (Gruber 1992) concludes that, while a thermal bremsstrahlung with an e-folding energy = 41.13 keV accurately fits the data up to 60 keV, above this energy the sum of two power laws is required with normalizations such that at 60 keV the spectral index is ~ 1.6, gradually flattening to ~ 0.7 at MeV energies. It should also be noted that below 10 keV the XRB energy spectrum is well represented by a power law of index α = 0.4 (I ∝ E−α).

Type
Part I: Invited Reviews
Information
Symposium - International Astronomical Union , Volume 168: Examining the Big Bang and the Diffuse Background Radiations , 1996 , pp. 263 - 270
Copyright
Copyright © Kluwer 1996 

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