The role of free neutrons in dynamics and radiation of Gamma-Ray Burst shocks

We show that presence of free neutrons in relativistic outflows in Gamma-Ray Burst (GRB) sources alters dynamical and radiative properties of these outflows. We review various neutron-related phenomena at different phases of fireball evolution and analyze their observable consequences. The most prominent effects are the following. At the fireball acceleration phase, there occurs the decoupling of proton and neutron components of GRB fireballs, which in some cases results in the neutrino catastrophe accompanied by a powerful neutrino signal. In the presence of intense radiation fields, produced by GRBs, continuous photon-induced conversion of protons to neutrons and back becomes possible. Such a conversion allows for efficient regime of particle acceleration through charge-changing nucleon reactions, where accelerated particles increase their energies in each particle-shock encounter by a factor roughly equal to the bulk Lorentz factor squared. This leads not only to copious production of highest-energy cosmic rays and neutrinos, but also serves as an efficient means of converting the energy of bulk motion into gamma-ray radiation. Another interesting effect is the broadening of the beam pattern of GRB jets at the highest photon energies, which can allow detection of off-axis GRBs by GLAST or other future GeV observatories.