Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption

We discuss expectations for the absorption of high-energy gamma-rays by γγ pair production with intergalactic radiation fields (IRFs) at very high redshifts (z ~ 5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-z observations including quasi-stellar object spectral measurements, Wilkinson Microwave Anisotropy Probe Thomson depth constraints, near-infrared source count limits, etc. We find that the ultraviolet IRF below the Lyman edge energy with intensities in the range of a few times 10-19ergcm-2s-1Hz-1sr-1 can cause appreciable attenuation above ~12 GeV at z ~ 5, down to ~6-8 GeV at z >~ 8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next-generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization.