GeV and higher energy photon interactions in gamma-ray burst fireballs and surroundings

We have calculated the opacities and secondary production mechanisms of high-energy photons arising in gamma-ray burst internal shocks, using exact cross sections for the relevant processes. We find that for reasonable choices of parameters, photons in the range of tens to hundreds of GeV may be emitted in the prompt phase. Photons above this range are subject to electron-positron pair production with fireball photons and would be absent from the spectrum escaping the gamma-ray burst. We find that, in such cases, the fireball becomes optically thin again at ultrahigh energies (≲PeV). On the other hand, for sufficiently large fireball bulk Lorentz factors, the fireball is optically thin at all energies. Both for γγ self-absorbed and optically thin cases, the escaping high-energy photons can interact with infrared and microwave background photons to produce delayed secondary photons in the GeV-TeV range. These may be observable with GLASTor at low redshifts with ground-based air Cerenkov telescopes. Detection of the primary prompt spectrum constrains the bulk Lorentz factor, while detection of delayed secondary gamma rays would provide a consistency check for the primary spectrum and the bulk Lorentz factor, as well as constraints on the intergalactic magnetic field strength.