1999MNRAS.307L..41T

The extreme energetics of the gamma-ray burst (GRB) 990123 have revealed that some GRBs emit quite a large amount of energy, and the total energy release from GRBs seems to change from burst to burst by a factor of 10²-10³ as E_γ,iso∼10^52–55erg, where E_γ,iso is the observed GRB energy when the radiation is isotropic. If all GRBs are triggered by similar events, such a wide dispersion in energy release seems odd. Here we propose a unified picture for the energetics of GRBs, in which all GRB events release roughly the same amount of energy E_iso∼10^55–56erg as relativistic motion, with the baryon load problem almost resolved. A mild dispersion in the initial Lorentz factor (Γ) results in a difference of E_γ,iso by up to a factor of m_pm_e∼10³. Protons work as `a hidden energy reservoir' of the total GRB energy, and E_γ,iso depends on the energy transfer efficiency from protons into electrons (or positrons) in the internal shock. We show that this transfer occurs via e^± pair creation by very high-energy photons of proton-synchrotron radiation, and this efficiency depends quite sensitively on Γ. We also show that, in spite of the wide dispersion in E_γ,iso, this model predicts a roughly constant photon energy range of the e^±-pair synchrotron at ∼MeV energies, which is nicely consistent with GRB observations. The optical flash of GRB 990123 can be explained by the internal shock origin in our model. The apparent non-correlation between E_γ,iso and the observed afterglow luminosity is consistent with the predictions of our scenario.