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Published online by Cambridge University Press: 14 August 2015
A review is given of the physical properties of extragalactic X-ray sources. The generation of X-rays as thermal bremsstrahlung, by the synchrotron mechanism or by Compton Scattering, is discussed. It is shown that each may be important depending on the circumstances. Only more detailed observations will enable us to decide which process dominates in any given source.