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Fireballs of gamma-ray bursts are partially made of free neutrons. Theirpresence crucially changes the mechanism of the fireball deceleration by anexternal medium. At late stages of the explosion, neutrons fully decouple fromions and coast with a constant Lorentz factor \Gamma_n. As the ion fireballdecelerates, the neutrons form a leading front. This front gradually decays,leaving behind a trail of decay products mixed with the ambient medium. Thekinetic energy of the decay products far exceeds the medium rest-mass energy,and the trail has a high Lorentz factor at radii R < R_{trail} = 10R_\beta\approx 10^{17} cm where R_\beta = 10^{16}(\Gamma_n/300) cm is the mean radiusof neutron decay. The ion fireball sweeps up the trail material and drives ashock wave in it. Thus, observed afterglow emission is produced in the neutrontrail. It can naturally re-brighten or display a spectral transition at R\approx R_{trail} where the impact of neutrons turns off. Absence of anyneutron signatures would point to an extremely low baryon loading of thefireballs and a strong dominance of a Poynting flux.Comment: accepted to ApJ Letter

Neutron-fed Afterglows of Gamma-Ray Bursts