  I show here that the time-delay between the gamma ray burst proper and the
onset of the afterglow is a sensitive function of the surrounding density,
going from ~10s for typical disk densities to several hours for IGM densities.
Since bulk Lorenz factor, explosion energy and environment density can be
simultaneously determined from observations of the time-delay and of the
afterglow, 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 merger hypothesis to a definitive test. If
bursts are due to collapse of massive stars, no matter how short the burst, the
onset of the afterglow should be immediate.
