3.6.3. Hadronic Processes

Hadronic processes refer to radiation from protons that are present in the GRB outflow [207–214]. These protons could in principle radiate via the proton synchrotron process and produce the observed gamma-ray prompt emission. However, even if these protons do not produce the observed gamma-ray spectrum, if present, they could potentially interact with photons and decay to pions through the delta resonance: photopion

processes (see, e.g., [207]). Pions decay into neutrinos, making GRBs possible sources of neutrinos; into leptons, which could in turn undergo synchrotron emission, and neutral pions decay directly to two high energy photons. Proton-photon interactions can also generate electron-positron pairs directly via the Bethe–Heitler process. Several authors have used the observed 100 MeV LAT prompt emission to constrain hadronic models [209–211,213]. To explain the 100 MeV LAT photons during the prompt phase, (i) the photopion and Bethe–Heitler processes require energy in protons larger than the observed gamma-ray energy by a factor of a thousand or more and (ii) the proton synchrotron mechanism requires protons to have a minimum Lorentz factor of <sup>∼</sup>10<sup>6</sup> , which is much larger than expected if the protons are accelerated in shocks [213]. This makes hadronic processes less energetically viable than leptonic models.
