Extended X-ray emission from radio galaxy cocoons

Abstract

We study the emission of X-rays from lobes of FR-II 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 $\gamma \ge 10^3$. This happens because of the concurrence of two times scales, that of the typical jet lifetimes and cooling due to inverse Compton scattering ($\sim 10^{7–8}$ yr), of electrons responsible for scattering CMB photons into keV range photons (with $\gamma \sim \sqrt{1\mathrm{keV}/k{T}_{CMB}}$). 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 $\propto (1+z{)}^{3.8}$, for typical values of parameters. We then estimate the X-ray luminosity function of FR-II radio galaxies and estimate the number of these diffuse X-ray bright objects above a flux limit of $\sim 3\times 10^{-16}$ erg cm${}^{-2}$ s${}^{-1}$ to be $\sim 25$ deg${}^{-2}$.

9 pages, 5 figures, accepted for publication in MNRAS (replaced with final version to match the printed version)

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• Decoding the giant extragalactic radio sources
• RAiSE X: searching for radio galaxies in X-ray surveys
• X-ray emission around the z=4.1 radio galaxy TNJ1338-1942 and the potential role of far-infrared photons in AGN Feedback
• Non-thermal Sunyaev-Zeldovich signal from radio galaxy lobes
• The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution
• From Nearby Low Luminosity AGN to High Redshift Radio Galaxies: Science Interests with SKA
• The non-thermal secondary CMB anisotropies from a cosmic distribution of radio galaxy lobes