Naked-eye optical flash from GRB 080319B: Tracing the decaying neutrons in the outflow

For an unsteady baryonic gamma-ray burst (GRB) outflow, the fast and slow proton shells collide with each other and produce energetic soft gamma-ray emission. If the outflow has a significant neutron component, the ultra-relativistic neutrons initially expand freely until decaying at a larger radius. The late time proton shells ejected from the GRB central engine, after powering the regular internal shocks, will sweep these $\beta-$decay products and give rise to very bright UV/optical emission. The naked-eye optical flash from GRB 080319B, an energetic explosion in the distant universe, can be well explained in this way.