Nucleosynthesis in gamma-ray burst accretion disks

We follow the nuclear reactions that occur in the accretion disks of stellar-mass black holes that are accreting at a very high rate, 0.01-1 M-circle dot s(-1), as is realized in many current models for gamma-ray bursts (GRBs). The degree of neutronization in the disk is a sensitive function of the accretion rate, black hole mass, Kerr parameter, and disk viscosity. For high accretion rates and low viscosity, material arriving at the black hole will consist predominantly of neutrons. This degree of neutronization will have important implications for the dynamics of the GRB-producing jet and perhaps for the synthesis of the r-process. For lower accretion rates and high viscosity, as might be appropriate for the outer disk in the collapsar model, neutron-proton equality persists, allowing the possible synthesis of Ni-56 in the disk wind. Ni-56 must be present to make any optically bright Type I supernova and, in particular, those associated with GRBs.