Identifying Nuclear Isomer Signatures in r-process Nucleosynthesis from Neutron Star Mergers

The landmark detection of the GW170817 binary neutron star (BNS) merger and its associated GRB 170817A gamma-ray burst revealed a hard, short pulse in the spectrum, which remains incompletely understood. This emission could arise from gamma-ray de-excitations of nuclear isomers, produced via r-process nucleosynthesis in the low-opacity ejecta exposed to the jet funnel. Our prior study identified three isomers—90Zr, 207Pb, and 89Y—as possible contributors. Here, we explore the need to include these isomers explicitly in nucleosynthesis models, using the PRISM nuclear reaction network and simulation trajectories from numerical relativity codes to quantify heavy nuclei production in the polar jet funnel. This enables a refined estimation of isomeric abundances and their energy spectra, as well as an assessment of multipolarity signatures in emitted gamma-rays from dominant isomers. Our goal is to accurately predict nuclear isomeric contributions to GRB emissions, enhancing our understanding of the nuclear processes in these astrophysical events.