SIMBAD references

We calculate the creation of electron-positron pairs in gamma-ray bursts (GRBs) resulting from the collision between scattered and outward-moving gamma-ray photons. The number of pairs exceeds the number of ambient-medium electrons encountered by the GRB ejecta up to ∼10¹⁶ cm from the centre of explosion. The shock resulting from the interaction of the ejecta with the pair wind may increase the afterglow synchrotron emission peak flux, in the optical band, during the first few minutes by a factor of less than about 5. The peak intensity of the optical afterglow increases with the density of the surrounding medium. Therefore, observations of the optical flux at early times constrain the density of the circumburst medium. If the electron and magnetic field energies behind the forward shock sweeping up the pair wind and the circumburst medium are as inferred from fits to the broad-band afterglow emission at 0.5-100 d, then the current upper limits on the optical counterpart emission, set by the ROTSE and LOTIS experiments, indicate that the circumburst medium within 0.01 pc is less dense than 100/cm3 or, if a wind, corresponds to a progenitor mass-loss to wind-speed ratio below 10^–6M_☉/yr/(10³km/s).