Probing high-redshift radiation fields with gamma-ray absorption

The next generation of gamma-ray telescopes may be able to observe gamma-ray blazars at high redshift, possibly out to the epoch of reionization. The spectrum of such sources should exhibit an absorption edge due to pair production against UV photons along the line of sight. One expects a sharp drop in the number density of UV photons at the Lyman edge epsilon (L). This implies that the universe becomes transparent after gamma-ray photons redshift below E similar to (m(e)c(2))(2)/epsilon (L)similar to 18 GeV. Thus, there is only a limited redshift interval over which GeV photons can pair produce. This implies that any observed absorption will probe radiation fields in the very early universe, regardless of the subsequent star formation history of the universe. Furthermore, measurements of differential absorption between blazars at different redshifts can cleanly isolate the opacity due to UV emissivity at high redshift. An observable absorption edge should be present for most reasonable radiation fields with sufficient energy to reionize the universe. Ly alpha photons may provide an important component of the pair production opacity. Observations of a number of blazars at different redshifts will thus allow us to probe the rise in comoving UV emissivity with time.