Pair Creation by Very High Energy Photons in Gamma-Ray Bursts: A Unified Picture for Various Energetics of GRBs

Abstract

The extreme energetics of the gamma-ray burst (GRB) 990123 has revealed that some of GRBs emit quite a large amount of energy, and total energy release from GRBs seems to change from burst to burst by a factor of 10^{2-3} as E_{gamma, iso} \sim 10^{52-55} erg, where E_{gamma, iso} is the observed GRB energy when 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 these various energetics of GRBs, in which all GRB events release roughly the same amount of energy of E_{iso} \sim 10^{55-56} erg as relativistic motion, with the baryon load problem almost resolved. A mild dispersion in the initial Lorentz factor (\Gamma) results in difference of E_{gamma, iso} up to a factor of m_p/m_e \sim 10^3. Protons work as ``a hidden energy reservoir'' of the total GRB energy, and E_{gamma, 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 \Gamma. We also show that, in spite of the wide dispersion in E_{gamma, iso}, this model predicts roughly a constant photon energy range of the e^{+-} pair-synchrotron at \sim MeV, which is well consistent with GRB observations. The optical flash of GRB 990123 can be explained by the internal shock origin in our model. The apparent no-correlation between E_{gamma, iso} and observed afterglow luminosity is consistent with the expectation of our scenario.