2001ApJ...560L.167P

The energy release in gamma-ray bursts (GRBs) is one of the most useful clues to the nature of their ``inner engines.'' We show that within the framework of the relativistic external shock afterglow model, the narrowness of the observed X-ray flux of GRB afterglows implies that the energy of the GRB jets, after the early-afterglow phase, spans less than 1 order of magnitude. This result is not affected by uncertainties in the electron energy, magnetic field strength, or external medium density. We argue that the afterglow kinetic energy is within a factor of 2 of the initial energy in the relativistic ejecta; therefore, the energy output of the central engine of a long-duration GRB has a universal value.