Non-thermal emission in the lobes of radio galaxies.

Radio and gamma-ray measurements of radiogalaxy lobes are useful to determine whether emission in these widely separated spectral regions is mainly by non-thermal (NT) electrons. This is of interest as there is yet no proof for a significant emission component from pion decay following NT proton interactions in the ambient lobe gas. An assessment of the hadronic yield needs full accounting of the local (FGL) and background (EBL, CMB) radiation fields in the lobes. Assuming a truncated single-PL electron energy distribution, exact calculation of the emission by NT electrons in the magnetized plasma in the Fornax A lobes leads to the conclusion that its Fermi-LAT emission is mostly IC/GFL: this result weakens earlier conclusions on the hadronic origin of the LAT emission. Similar analyses of the lobe emissions of Cen A, Cen B, and NGC 6251 suggest their measured LAT emissions, too, to be of IC/(EBL, CFGL, CMB) nature. Measured emissions of distant radio-galaxy lobes (3C98, Pictor A, DA240, Cygnus A, 3C326, and 3C236) are currently limited to the radio and X-ray bands: they can give no information on the presence of NT protons, but do trace the properties of NT electrons, and allow calculations of the related IC gamma-ray emission to be performed. The e/B energy density ratios, U_e/U_B, turn out to be in the range ~1-100. The NT proton energy density, U_p, is spectrally constrained to be less than a few tens of eV/cm3. Despite this limit, arguably U_p >> U_e -- as suggested by arguments of lobe internal vs external pressure. Thus the lobes' NT energy budget is likely dominated by particles. Given the low thermal energy densities measured in lobes, NT energy dominance is probably a general feature of lobe energetics

High-energy emission from star-forming galaxies

Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, the steady-state energy distributions of these particles in the starburst galaxies M82 and NGC253 can be determined with a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region, and a radial profile is assumed for the magnetic field. The resulting radiative yields of electrons and protons are calculated: the predicted >100MeV and >100GeV fluxes are in agreement with the corresponding quantities measured with the orbiting Fermi telescope and the ground-based VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in central regions of starburst galaxies, as inferred from the radio and gamma-ray measurements of (respectively) non-thermal synchrotron and neutral-pion-decay emission, are U=O(100) eV/cm3, i.e. at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. These very different energy density levels reflect a similar disparity in the respective supernova rates in the two environments. A L(gamma) ~ SFR^(1.4) relationship is then predicted, in agreement with preliminary observational evidence.Comment: Invited talk at SciNeGHE2010 (8th Wotkshop on Science with the New Generation of High Energy Gamma-ray Experiments): Gamma-ray Astrophysics in the Multimessenger Context (Trieste, Sept.8-10, 2010

Very-High Energy Gamma Astrophysics

High-energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge of the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high-energy gamma-rays. Observation of gamma-rays gives an exciting view of the high-energy universe thanks to satellite-based telescopes (AGILE, GLAST) and to ground-based detectors like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which recently discovered more than 60 new very-high-energy sources. The progress achieved with the last generation of Cherenkov telescopes is comparable to the one drawn by EGRET with respect to the previous gamma-ray satellite detectors. This article reviews the present status of high-energy gamma astrophysics, with emphasis on the recent results and on the experimental developments.Comment: 60 pages, 52 figures, (on line abstract replacement