Anisotropic inverse Compton scattering from the trans–relativistic to the ultra–relativistic regime and application to the radio galaxies

The problem of the anisotropic Inverse Compton scattering between a monochromatic photon beam and relativistic electrons is revisited and formally solved without approximations. Solutions are given for the single scattering with an electron beam and with a population of electrons isotropically distributed, under the assumption that the energy distribution of the relativistic particles follows a simple power law as it is the case in many astrophysical applications. Both the Thomson approximation and the Klein-Nishina regime are considered for the scattering of an unpolarized photon beam. The equations are obtained without the ultra–relativistic approximation and are compared with the ultra–relativistic solutions given in the literature. The main characteristics of the power distribution and spectra of the scattered radiation are discussed for relevant examples. In the Thomson case for an isotropic electron population a simple formula holding down to the mildly–relativistic energies is given. As an application the formulae of the anisotropic inverse Compton scattering are used to predict the properties of the X and γ–ray spectra from the radio lobes of strong FR II radio galaxies due to the interaction of the relativistic electrons with the incoming photons from the nucleus. The dependence of the emitted power on the relativistic electron energy distribution and on its evolution with time is discussed. PACS: 95.30.Cq; 95.30.Gv; 98.38.Am; 98.54.Gr; 98.70.Qy