Fireballs with a Neutron Component

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.