The gamma-ray horizon

We discuss several effects due to absorption of high energy gamma-rays by pair production with ambient background photons. This absorption process leads to the concept of the gamma-ray horizon, which is the energy dependent distance beyond which the optical depth due to this process exceeds unity. At low redshift we investigate the cut-off in the spectrum of Mkn501 $(z=0.034),$ which is observed to be in the vicinity of $Eγ=10 TeV.$ To date there are no GeV-TeV sources with well established spectra and redshifts larger than that of Mkn501. Therefore we can only consider absorption for hypothetical sources as a function of redshift, to predict the locii of the spectral cut-offs for various redshifts. Our study shows that absorption of photons above 50 GeV is severe for redshifts beyond ≈3. Thus to sample high energy sources to even larger redshifts will require observations well below 50 GeV. In this energy regime the GLAST experiment, to be launched in 2005, will provide a significant improvement in sensitivity over previous missions. Observations from the ground with the next generation of Cherenkov telescopes (MAGIC, VERITAS, HESS …) will also provide increased sensitivity and, more importantly, lower thresholds. The combined effort of space- and ground-based high energy observatories is expected to provide data on a large sample of GeV/TeV sources, which will shed light on the central engines of active galaxies and probe the metagalactic radiation field through the pair creation opacity. High energy radiation from a variety of sources throughout the universe accumulates to the diffuse extragalactic gamma-ray background, as measured by EGRET. The spectrum of this background is also effected by absorption due to pair production. Our study shows that this effect occurs above ≈100 GeV, just beyond the reach of the EGRET experiment.