Abstract
A new class of source appears to be necessary to explain the X-ray background. We outline a candidate non-thermal spectral mechanism in which the electron acceleration process is ‘loaded’ by pairs produced in the source by photon–photon collisions. This identifies the break energy, which is now around 40 keV, with the electron rest-mass, 511 keV. The new sources are very distant objects at |$z\sim 10-30$| and pose some problems for Cold Dark Matter theories of cosmic structure formation.
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