INVERSE COMPTON X-RAYS FROM STRONG FRII RADIO-GALAXIES

In the framework of the quasars-radio galaxies unification the radio-e mitting lobes of FRII radio galaxies are pervaded by an intense quasar radiation field. Inverse Compton (IC( scattering between the relativi stic electrons and the IR-optical photons from a hidden quasar may pro vide an important contribution to the X-ray emission of these radio ga laxies. The soft X-ray emission properties of six strong, high redshif t FRIIs (3C 277.2, 280, 294, 324, 356, 368) are compared with our mode l expectations, taking into account also the contribution from the IC scattering of the CMB photons with the radio electrons. Our estimates are based on a typical quasar spectrum, derived from the infrared and optical properties of a 3C quasar sample, and on the assumption of ene rgy equipartition between relativistic particles and magnetic fields w ith the same energy density in the electron and proton components and with a fixed low energy cut-off in the particle distribution (Appendix A). We find that the soft X-ray luminosities and spectra of five out of six sources can be satisfactorily explained by our model with the e xception of 3C 324 whose X-ray emission is probably dwarfed by that of the galaxies' cluster of which this source is a member. In the case o f 3C 277.2 our model requires a luminosity of the hidden quasar which is in perfect agreement with that derived from spectropolarimetric stu dies. In order to carry out the computations of the IC scattering of t he hidden quasar photons, which an propagating radially outward, we ha ve solved the anisotropic IC problem. The formal approach and relevant formulae, which do not appear to be available in the literature, are presented in the Appendix B. One important effect is the prediction th at the observed X-ray emission associated with the two radio lobes wou ld be asymmetric if the radio axis is inclined with respect to the pla ne of the sky, the far-away lobe being the more luminous. The ratio be tween the X-ray luminosities of the two lobes is derived for several v alues of the inclination of the radio axis. However, the predicted ang ular sizes of these distant radio galaxies are small and difficult to be resolved with present X-ray facilities.