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Astron. Astrophys. 325, 898-910 (1997)

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Inverse Compton X-rays from strong FRII radio-galaxies

G. Brunetti 1, 2, G. Setti 1, 2 and A. Comastri 3

1 Dipartimento di Astronomia, via Zamboni 33, I-40126 Bologna, Italy
2 Istituto di Radioastronomia del CNR, via Gobetti 101, I-40126 Bologna, Italy
3 Osservatorio Astronomico di Bologna, via Zamboni 33, I-40126 Bologna, Italy

Received 1 November 1996 / Accepted 17 March 1997


In the framework of the quasars-radio galaxies unification the radio-emitting lobes of FRII radio galaxies are pervaded by an intense quasar radiation field. Inverse Compton (IC) scattering between the relativistic electrons and the IR-optical photons from a hidden quasar may provide an important contribution to the X-ray emission of these radio galaxies. The soft X-ray emission properties of six strong, high redshift FRIIs (3C 277.2, 280, 294, 324, 356, 368) are compared with our model 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 energy equipartition between relativistic particles and magnetic fields with 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 exception 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 of 3C 277.2 our model requires a luminosity of the hidden quasar which is in perfect agreement with that derived from spectropolarimetric studies. In order to carry out the computations of the IC scattering of the hidden quasar photons, which are propagating radially outward, we have 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 that the observed X-ray emission associated with the two radio lobes would be asymmetric if the radio axis is inclined with respect to the plane of the sky, the far-away lobe being the more luminous. The ratio between the X-ray luminosities of the two lobes is derived for several values of the inclination of the radio axis. However, the predicted angular sizes of these distant radio galaxies are small and difficult to be resolved with present X-ray facilities.

Key words: radiation mechanism: non-thermal – scattering – galaxies: active – quasars: general – X-rays: general

Send offprint requests to: G. Brunetti, Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998