High Energy gamma-rays From FR I Jets

Abstract

Thanks to 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 a lot of uncertainties - connected for example with the particle acceleration processes involved - remain. In this paper we study production of high energy gamma-rays in FR I kiloparsec-scale jets by inverse-Compton emission of the synchrotron-emitting electrons. We consider different origin 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 non-detections of the evaluated gamma-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 M 87, we find measurable fluxes of TeV photons resulting from synchrotron self-Compton process and from comptonisation of the galactic photon fields, respectively. In the case of Centaurus A, we also find a relatively strong emission component due to comptonisation of the nuclear blazar photons, which could be easily observed by GLAST at energy ~10 GeV, providing important test for the unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for publication in Astrophysical Journa