Nuclear Isomers and Their Impact on Gamma-Ray Emission in Binary Neutron Star Mergers

Abstract

The multi-messenger observations of GW170817 have provided strong evidence that binary neutron star mergers are a site of heavy-element production through the rapid neutron capture process. The decay of unstable $r$-process nuclei produces a significant number of $\gamma$-rays, which not only power the associated kilonova, but may also be observable in current and future $\gamma$-ray observatories, providing a direct probe of the nuclei synthesized. Current models of the emitted $\gamma$-rays link a nuclear reaction network with individual decay spectra. Typically, only the population of the ground state of the nuclei is tracked in the reaction network, and rapid de-excitation of the daughter nuclei is assumed when calculating the decay spectra. This may not be accurate in nuclei where there are longer-lived, high-energy nuclear isomers. In this work, we relax these assumptions, modeling the dynamic behavior of nuclear isomers for a select list of nuclei. These isomers are included as independent states in the network, with temperature-dependent effective transition rates between ground and isomeric states and $\beta$-feeding probabilities calculated using the nuclear level structure. We estimate the $\gamma$-ray flux from isomeric transitions, finding that the 743.3 keV line of Nb-97m and the 555.6 keV line of Y-91m may be detectable in $\gamma$-ray observatories such as COSI, AMEGO, and LOX at galactic distances. These results, calculated for a small fraction of nuclei with isomers, highlight the need for the robust inclusion of nuclear isomers in nuclear reaction networks, as well as careful modeling of the resultant $\gamma$-ray spectroscopy to characterize observational signals.