Beacons at the Gamma Ray Horizon

Blazars with redshifts z ≤ 0.1 are likely candidates for detection at energies in the range 300 GeV – 50 TeV withČerenkov telescopes and scintillator arrays. We present γ-ray flux predictions for a sample of 15 nearby flat-spectrum radio sources fitting the proton blazar model of Mannheim (1993a) to their observed broad-band spectral energy distributions. At high energies, we use fluxes or flux limits measured by ROSAT, CGRO and the Whip-ple Observatory to constrain their spectra. We take into account absorption of the γ-rays by pair production with low energy photons of the diffuse infrared-to-optical pho-ton background produced by galaxies (cosmic absorption) and with low energy synchrotron photons of the blazar radiation field (internal absorption). Typically, the theoretical spectra decrease much faster above TeV (photon index s ≈ 3) than between GeV and TeV (s ≈ 2) owing to internal absorption. The predicted fluxes are confronted with flux limits in the 20-50 TeV energy range obtained by the High Energy Gamma Ray Astronomy (HEGRA) experiment. Without cosmic absorption, the fluxes are about equal to the current sensitivity of HEGRA. Improved γ/hadron separation techniques could render a detection by HEGRA possible , if cosmic absorption by the far-infrared background at wavelengths ∼ 100 µm is not exceedingly strong.