Within the internal shock scenario we consider different mechanisms of high energy ( > 1 MeV ) 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 fraction of shock energy carried by electrons is not very small ( e.g . \epsilon _ { e } > 10 ^ { -3 } ) .
For very small values of \epsilon _ { e } the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime .
However , in this case 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 a canonical fireball bulk Lorentz factor ( e.g . \Gamma = 400 ) , emissions above \sim 10 GeV are attenuated by two-photon pair production processes .
For a fireball with an even higher Lorentz factor , the cutoff energy is higher , and emissions of 10 TeV - PeV due to \pi ^ { 0 } -decay can also escape from the internal shocks .
The flux level is however too low to be detected by current TeV detectors , and these photons also suffer attenuation by external soft photons .
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 ” durations ( e.g . \sim 1 s ) .
With the observed high energy spectrum alone , it appears that there is no clean picture to test the leptonic vs. hadronic origin of the 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 .