Heating and deceleration of GRB fireballs by neutron decay

Fireballs with high energy per baryon rest mass (η > η∗ ∼ 400) contain a relatively slow neutron component. We show here that in this situation the thermal history of fireballs is very different from the standard adiabatic cooling. INTRODUCTION Baryonic matter ejected in a GRB explosion is partially composed of free neutrons (see [1] and [2] for detailed calculations and references). This has a strong impact on the external blast wave and changes the afterglow mechanism at radii r < ∼ 10 17 cm [3]. We here investigate the dynamical effect of neutrons on the early evolution of a relativistic fireball, prior to the afterglow phase. FIREBALL ACCELERATION AND NEUTRON-PROTON DECOUPLING At the initial stage of the fireball acceleration, the neutrons are collisionally coupled to the protons and accelerated by radiation pressure at the same rate. If the energy-perbaryon η is high enough, neutrons decouple before the acceleration stage is completed. This happens if