Prompt Emission of High Energy Photons from Gamma Ray Bursts

Within the internal shock scenario we consider different mechanisms of high energy photon production inside a Gamma Ray Burst (GRB) fireball and derive the expected high energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high energy emission is dominated by the leptonic component if energy carried by electrons is not unreasonably small (e.g. ǫe > 10). For very small values of ǫe the hadronic emission component may be comparable to the leptonic component in the GeV-TeV regime. However, a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For canonical bulk Lorentz factors of the fireballs (e.g. Γ = 400), emissions above ∼ 10 GeV are attenuated by two-photon pair production processes. For even higher Lorentz factor fireballs, emissions of 10 TeV PeV due to π-decay can escape from the internal shocks, but they are generally attenuated by external soft photons. The reprocessed emission would contribute to the diffuse gamma-ray background. GLAST LAT can detect prompt emission of bright long GRBs above 100 MeV. For short GRBs, the prompt emission can be only barely detected for nearby bright ones with relatively “long” duration (e.g. ∼ 1 s). With the observed high energy spectrum alone, it appears that there is no clean signature to test the leptonic vs. hadronic origin of gamma-rays. Such an issue may be however addressed by collecting both prompt and afterglow data. A moderate-to-high radiative efficiency would suggest a leptonic origin of high energy photons, while a GRB with an extremely low radiative efficiency but an extended high energy emission component would be consistent with (but not a proof for) the hadronic origin.