Probing High‐Redshift Radiation Fields with Gamma‐Ray Absorption

The next generation of gamma-ray telescopes may be able to observe gamma-rayblazars at high redshift, possibly out to the epoch of reionization. Thespectrum of such sources should exhibit an absorption edge due topair-production against UV photons along the line of sight. One expects a sharpdrop in the number density of UV photons at the Lyman edge E_{L}. This impliesthat the universe becomes transparent after gamma-ray photons redshift below E(m_{e}c^2)^{2}/E_{L} 18 GeV. Thus, there is only a limited redshift intervalover which GeV photons can pair produce. This implies that any observedabsorption will probe radiation fields in the very early universe, regardlessof the subsequent star formation history of the universe. Furthermore,measurements of differential absorption between blazars at different redshiftscan cleanly isolate the opacity due to UV emissivity at high redshift. Anobservable absorption edge should be present for most reasonable radiationfields with sufficient energy to reionize the universe. Ly-alpha photons mayprovide an important component of the pair-production opacity. Observations ofa number of blazars at different redshifts will thus allow us to probe the risein comoving UV emissivity with time.Comment: ApJ accepted version, minor changes. 19 pages, 5 figure