Reflection-dominated hard X-ray sources and the X-ray background

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Abstract
The spectrum of the X-ray background is flat in the 2–10 keV range and has a break at about 30 keV. We shown here that the spectrum of hard X-rays scattered by Thomson-thick material gives a good fit to the background above 10 keV when redshifted to z ~ 1.5. An evolutionary model covering a range of redshifts explains the background spectrum over a wider energy band. The necessary reflection-dominated sources may occur when massive black holes grow by accretion. A large fraction of the power generated by the sources must emerge as thermal radiation in the EUV and may be the intergalactic ionizing flux required to explain the properties of QSO absorption lines.

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