New connection between central engine weak physics and the dynamics of gamma-ray burst fireballs

We demonstrate a qualitatively new aspect of the dynamics of gamma-ray burst (GRB) fireballs: the development of a substantial dispersion in the proton component in fireballs in which neutron decoupling occurs and is sufficiently pronounced. This effect depends sensitively on the neutron to proton ratio in the fireball, becoming more dramatic with increasing neutron excess. Simple physical arguments and transport calculations indicate that the dispersion in the Lorentz factor of the protons can be of the order of the final mean Lorentz factor of the fireball. We show how plasma instabilities could play an important role in the evolution of the fireball and how they might ultimately govern the development of such a velocity dispersion in the proton component. The role of these instabilities in setting or diminishing a proton Lorentz factor dispersion represents a new and potentially important venue for the study of plasma instabilities. Significant dispersion in the proton velocities translates into fewer protons attaining the highest Lorentz factors. This is tantamount to a reduction in the total energy required to attain a given Lorentz factor for the highest energy protons. As well, a velocity dispersion in the proton component can have consequences for the electromagnetic and neutrino signature of GRB’s.