Modeling Nuclear Isomers as Sources of Gamma Rays in Binary Neutron Star Mergers

Outflows from binary neutron star (BNS) mergers provide astrophysical environments for rapid neutron capture (r-process) nucleosynthesis, where heavy nuclei are formed in highly excited states. Some of these nuclei become trapped in long-lived isomeric states that de-excite through distinctive gamma-ray emission. Because the r-process evolves in sync with a gamma-ray burst (GRB) jet, certain isomers can remain confined within magnetically trapped blobs of matter expelled through the jet-cleared funnel. Their gamma rays may escape earlier, producing spectral lines even before the afterglow phase. We propose nuclear isomers as a new source of gamma-ray emission in GRBs and explore their role using the PRISM nucleosynthesis framework with ejecta trajectories from numerical-relativity simulations. For peak r-process isomers, we first calculate temperature-dependent effective transition rates, then the isomeric abundances from PRISM, and finally the resulting luminosity and spectral signatures. Our findings highlight the importance of including nuclear isomers in r-process models and their potential to shape gamma-ray emission from merger-driven environments.