Inverse-Compton ghosts and double-lobed radio sources in the X-ray sky

In this study we predict the total distributions of powerful [Fanaroff and Riley Class II (FR II)] active double-lobed radio galaxies and ghost sources, and their observable distribution in the X-ray sky. We develop an analytic model for the evolution of the lobe emission at radio and X-ray energies. During jet activity, a double radio source emits synchrotron radiation in the radio and X-ray emission due to inverse-Compton (IC) upscattering by gamma similar to 103 electrons of the cosmic microwave background. After the jets switch off, the radio luminosity (due to higher gamma electrons) falls faster than the X-ray luminosity and for some time the source appears as an IC ghost of a radio galaxy before becoming completely undetectable in the X-ray. With our model, for one set of typical parameters, we predict radio lobes occupy a volume fraction of the Universe of 0.01, 0.03, 0.3 at z = 2 (during the quasar era) of the filamentary structures in which they are situated, for typical jet lifetimes 5 x 107 yr, 108 yr, 5 x 108 yr; however since the inferred abundance of sources depends on how quickly they fall below the radio flux limit the volume filling factor is found to be a strong function of radio galaxy properties such as energy index and minimum gamma factor of injected particles, the latter not well constrained by observations. We test the predicted number density of sources against the Chandra X-ray Deep Field North survey and also find the contribution to the unresolved cosmic X-ray background by the lobes of radio galaxies. 10-30 per cent of observable double-lobed structures in the X-ray are predicted to be IC ghosts. The derived X-ray luminosity function of our synthetic population shows that double-lobed sources have higher space densities than X-ray clusters at redshifts z >= 2 and X-ray luminosities above 1044 erg s-1.