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 densit y, going from approximate to 10 s for typical disk densities to several hou rs for IGM densities. I show that bulk Lorenz factor, explosion energy and environment density can be simultaneously determined from observations of t he time delay and of the afterglow. Thus, afterglow observations alone may establish which fraction, if any, of all bursts resides inside galaxies, in their outer haloes, or outright in the IGM medium, subjecting the NS/NS me rger hypothesis to a definitive test. If bursts are due to collapse of mass ive stars, no matter how short the burst, the onset of the afterglow should be immediate. (C) 2000 Elsevier Science B.V. All rights reserved.