2003ApJ...585L..19B

Fireballs of gamma-ray bursts are partially made of free neutrons. Their presence crucially changes the mechanism of the fireball deceleration by an external medium. At late stages of the explosion, neutrons fully decouple from ions and coast with a constant Lorentz factor Γ_n. As the ion fireball decelerates, the neutrons form a leading front. This front gradually decays, leaving behind a trail of decay products mixed with the ambient medium. The kinetic energy of the decay products far exceeds the medium rest-mass energy, and the trail has a Lorentz factor γ≫1 at radii R<R_trail~10R_β~10¹⁷ cm, where R_β~10¹⁶(Γ_n/300) cm is the mean radius of neutron decay. The ion fireball sweeps up the trail material and drives a shock wave in it. Thus, observed afterglow emission is produced in the neutron trail. It can naturally rebrighten or display a spectral transition at R~R_trail where the impact of neutrons turns off. The absence of any neutron signatures would point to an extremely low baryon loading of the fireballs and a strong dominance of a Poynting flux.