Extended X-ray emission from radio galaxy cocoons

We study the emission of X-rays from lobes of Fanaroff-Riley type II (FRII) radio galaxies by inverse-Compton scattering of microwave background photons. Using a simple model that takes into account injection of relativistic electrons, their energy losses through adiabatic expansion, synchrotron and inverse-Compton emission, and also the stopping of the jet after a certain time, we study the evolution of the total X-ray power, the surface brightness, angular size of the X-ray bright region and the X-ray photon index, as functions of time and cocoon size, and compare the predictions with observations. We find that the radio power drops rapidly after the stopping of the jet, with a shorter time-scale than the X-ray power. The X-ray spectrum initially hardens until the jet stops because the steepening of electron spectrum is mitigated by the injection of fresh particles, for electrons with gamma >= 10(3). This happens because of the concurrence of two times scales, that of the typical jet lifetimes and cooling due to inverse-Compton scattering (similar to 10(7-8) yr), of electrons responsible for scattering cosmic microwave background photons into keV range photons (with gamma similar to root 1keV/k(TCMB)). Another finding is that the ratio of the X-ray to radio power is a robust parameter that varies mostly with redshift and ambient density, but is weakly dependent on other parameters. We also determine the time-averaged ratio of X-ray to radio luminosities (at 1 keV and 151 MHz) and find that it scales with redshift as proportional to(1 + z)(3.8), for typical values of parameters. We then estimate the X-ray luminosity function of FRII radio galaxies and estimate the number of these diffuse X-ray bright objects above a flux limit of similar to 3 x 10(-16) erg cm(-2) s(-1) to be similar to 25 deg(-2).