Relativistic fireballs: energy conversion and time-scales

The expansion energy of a relativistic fireball can be reconverted into radiation when it interacts with an external medium. For expansion with Lorentz factors ≳ 103 into a typical galactic environment, the corresponding time-scale in the frame of the observer is of the order of seconds. This mechanism would operate in any cosmological scenario of gamma-ray bursts involving initial energies of order a per cent of a stellar rest mass, and implies photon energies and time-scales compatible with those observed in gamma-ray bursts.