Central engine weak physics and the role of neutrons in gamma ray burst fireballs

I derive implications of the neutron to proton ratio (n/p) in relativistic Gamma Ray Burst (GRB) fireballs for the dynamics of these fireballs and for the electromagnetic signature of GRBs. Also, the connection between the neutron to proton ratio in GRBs and the weak physics characterizing the GRB central engine is considered.; I discuss the setting of the neutron to proton ratio (or alternatively the electron fraction Y_e = p/(n + p)) in neutrino heated ultra-relativistic winds from compact objects. These winds are an idealization of the class GRB central engine models which invoke neutrino heating to drive an outflow. It is found that the number of neutrino captures per baryon in these winds is of order a few, so that the composition of the outflow likely mirrors conditions in the neutron star crust. Interestingly, these winds represent a new class of solutions to the steady state eigenvalue problem discussed in the context of the late time Type II supernovae. Whether or not this ultra-relativistic branch of the steady state solutions has implications for nucleosynthesis in late time supernovae remains to be studied.; Consequences of the ratio of neutrons to protons for the dynamics of GRB fireballs are discussed. It is shown that neutron decoupling, the process whereby neutrons dynamically decouple during the acceleration stage of a relativistic fireball's evolution, can induce a substantial dispersion in the proton component of the fireball. This dispersion may give rise to the generation of magnetic fields via plasma instabilities. More basically, the presence of a velocity dispersion in the charged particle component in the fireball means that collisional processes are not sufficient to describe the evolution of relativistic fireballs.; An electromagnetic signature of neutron decoupling and of the neutron to proton ratio in GRB lightcurves is proposed. It is shown that neutron rich outflows and neutron poor outflows give rise to dramatically different lightcurves. A preliminary analysis of existing BATSE data with an eye to inferring the neutron to proton ratio is presented.