High‐Energy Gamma Rays from FR I Jets

Thanks to the Hubble and Chandra telescopes, some of the large-scale jets in extragalactic radio sources are now being observed at optical and X-ray frequencies. For the FR I objects the synchrotron nature of this emission is surely established, although many uncertainties—connected for example with the particle acceleration processes involved—remain. In this paper we study the production of high-energy γ-rays in FR I kiloparsec-scale jets by inverse Compton emission of the synchrotron-emitting electrons. We consider different origins of seed photons contributing to the inverse Compton scattering, including nuclear jet radiation as well as ambient, stellar, and circumstellar emission of the host galaxies. We discuss how future detections or nondetections of the evaluated γ-ray fluxes can provide constraints on the unknown large-scale jet parameters, i.e., the magnetic field intensity and the jet Doppler factor. For the nearby sources Centaurus A and M87, we find measurable fluxes of TeV photons resulting from synchrotron self-Compton process and from Comptonization of the galactic photon fields, respectively. In the case of Centaurus A, we also find a relatively strong emission component due to Comptonization of the nuclear blazar photons, which could be easily observed by GLAST at energies ~10 GeV, providing an important test for the unification of FR I sources with BL Lacertae objects.