Absorption of Nuclear Gamma-rays on the Starlight Radiation in FR I Sources: the Case of Centaurus A

Abstract

Several BL Lac objects are confirmed sources of variable and strongly Doppler-boosted TeV emission produced in the nuclear portions of their relativistic jets. It is more than probable, that also many of the FR I radio galaxies, believed to be the parent population of BL Lacs, are TeV sources, for which Doppler-hidden nuclear gamma-ray radiation may be only too weak to be directly observed. Here we show, however, that about one percent of the total time-averaged TeV radiation produced by the active nuclei of low-power FR I radio sources is inevitably absorbed and re-processed by photon-photon annihilation on the starlight photon field, and the following emission of the created and quickly isotropized electron-positron pairs. In the case of the radio galaxy Centaurus A, we found that the discussed mechanism can give a distinctive observable feature in the form of an isotropic gamma-ray halo. It results from the electron-positron pairs injected to the interstellar medium of the inner parts of the elliptical host by the absorption process, and upscattering starlight radiation via the inverse-Compton process mainly to the GeV-TeV photon energy range. Such a galactic gamma-ray halo is expected to possess a characteristic spectrum peaking at ~0.1 TeV photon energies, and the photon flux strong enough to be detected by modern Cherenkov Telescopes and, in the future, by GLAST. These findings should apply as well to the other nearby FR I sources.

24 pages, 8 figures included. Accepted for publication in MNRAS

References in corpus (3)

• Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet
• Hard TeV spectra of blazars and the constraints to the IR intergalactic background
• Observations of Selected AGN with H.E.S.S

Cited by in corpus (12)

• High-energy particle acceleration at the radio-lobe shock of Centaurus A
• High-energy Particle Acceleration and Production of Ultra-high-energy Cosmic Rays in the Giant Lobes of Centaurus A
• Ultra High Energy Cosmic Rays from Black Hole Jets of Radio Galaxies
• A synchrotron self-Compton scenario for the very high energy gamma-ray emission of the radiogalaxy M87
• Resolving acceleration to very high energies along the Jet of Centaurus A
• TeV Gamma-ray Astronomy: A Summary
• Scrutinizing FR 0 Radio Galaxies as Ultra-High-Energy Cosmic Ray Source Candidates
• The Complex North Transition Region of Centaurus A: Radio Structure
• Curvature in the very-high energy gamma-ray spectrum of M87
• Physical Conditions and Particle Acceleration in the Kiloparsec Jet of Centaurus A
• Morphology of TeV gamma-ray emission from the kpc scale jets in radio galaxies
• Attenuation of TeV $\gamma$-rays by the starlight photon field of the host galaxy