Abstract
IT has recently been suggested that the isotropic X-ray background can be accounted for by means of the superposed contributions from extra-galactic X-ray sources, provided that evolutionary effects are taken into account1,2. The neglect of evolutionary factors leads to a diffuse X-ray flux from discrete extra-galactic sources which falls short of the observed flux by between one and two orders of magnitude3. This appears to be true whether one considers normal galaxies (P∼1040 ergs/s), radio galaxies (for M 87, P∼1043 ergs/s), or quasi-stellar sources (for 3C 273, P≲1046 ergs/s). Here P is the flux in X-rays above 1 keV, and the estimates of P are justified later.
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SILK, J. Discrete X-ray Sources and the Diffuse X-ray Background. Nature 221, 347–348 (1969). https://doi.org/10.1038/221347a0
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DOI: https://doi.org/10.1038/221347a0
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