The Evolution of Radio Sources: The Origin of Cosmic Rays and the X-ray Background

Three interrelated problems are discussed in this paper—the theory of the extragalactic origin of cosmic rays, the origin of the X-ray background and the evolution of extragalactic radio sources. Cosmic ray electrons originating in extragalactic radio sources produce not only radio waves by the synchrotron mechanism but also X-rays by inverse Compton scattering of the relict radiation. The latter is likely to be the dominant loss mechanism for electrons at large redshifts. There then exists a simple relation between the local flux of cosmic rays and the X-ray background. Using the conventional value. ℋ = 100 for the ratio. ℋ of energy densities in relativistic protons and electrons generated in radio sources, the upper limit to the local energy density of cosmic rays due to extragalactic radio sources is $${10}^{-3}\,\text{eV}\,\text{cm}^{-3}$$ . Only if ℋ > 10 ⁴ can the whole of the observed cosmic rays be attributed to extragalactic sources. Although the total energy associated with the X-ray background can be produced in this model, there are very severe difficulties in accounting for the break in the observed background spectrum at $${\epsilon}_{x}\,\sim\,40$$ . These arise because the available radio data give information about the injection spectra of cosmic ray electrons into the intergalactic medium. It is also shown that the apparent cut-off in the distribution of radio sources at large redshifts cannot be attributed to the effects of inverse Compton scattering.