The Energy Distribution of Long Duration GRBS

The energy release in gamma-ray bursts is one of the most interesting clues on the nature of their "inner engines". We show here that the total energy release in GRBs varies by less than one order of magnitude from one burst to another while the energy emitted in $\gamma$-ray photons varies by more than an order of magnitude. This result indicates that the central engine of long duration GRB has a remarkably constant energy output which provides very important constraint on the nature of these enigmatic explosions. The broader distribution of the observed $\gamma$-ray flux, about three orders of magnitude in width, can be attributed, in part, to a variation in the opening angle of the collimated explosion, and in part to the variation of Lorentz factor across the jet as well as a variable efficiency for converting the kinetic energy of explosion to $\gamma$-rays.