  Standard GRB fireballs must carry free neutrons. This crucially changes the
mechanism of fireball deceleration by an external medium. As the ion fireball
decelerates, the coasting neutrons form a leading front. They gradually decay,
leaving behind a relativistic trail of decay products mixed with the ambient
medium. The ion fireball sweeps up the trail and drives a shock wave in it.
Thus, observed afterglow emission is produced in the neutron trail. The impact
of neutrons turns off at 10^{17}cm from the explosion center, and here a
spectral transition is expected in GRB afterglows. Absence of neutron
signatures would point to absence of baryons and a dominant Poynting flux in
the fireballs.
