Nuclear composition of gamma-ray burst fireballs

We study three processes that shape the nuclear composition of the fireballs in gamma-ray bursts (GRBs): (1) neutronization in the central engine, (2) nucleosynthesis in the fireball as it expands and cools, and (3) spallation of nuclei in subsequent internal shocks. The fireballs are shown to have a neutron excess and a marginally successful nucleosynthesis. They are composed of free nucleons, α-particles, and deuterium. A robust result is the survival of a significant neutron component, which has important implications. First, as shown in previous works, neutrons can lead to observable multiGeV neutrino emission. Second, as we show in an accompanying paper, the neutrons impact the explosion dynamics at radii up to 1017 cm and change the mechanism of the GRB afterglow emission. Subject headings: accretion, accretion disks — cosmology: miscellaneous — dense matter — gamma rays: bursts — nuclear reactions, nucleosynthesis