A simple, stringent test on the nature of GRB progenitors

I show here that the time delay between the gamma ray burst proper and the onset of the afterglow to be a sensitive function of the surrounding density, going from ~10 s for typical disk densities to several hours for IGM densities. I show that bulk Lorenz factor, explosion energy and environment density can be simultaneously determined from observations of the lime delay and of the afterglow. Thus. afterglow observations alone may establish which fraction, if Aany. of all bursts resides inside galaxies, m their outer haloes, or outright in the IGM medium, subjecting the NS/NS merger hypothesis to a definitive test. If bursts are due lo collapse of massive stars, no matter how short the bursl, the onset of the afterglow should be immediate.