MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers

Neutron star mergers (NSMs) are the first verified site of rapid neutron capture ($r$-process) nucleosynthesis, and could emit gamma rays from the radioactive isotopes synthesized in the neutron-rich ejecta. These MeV gamma rays may provide a unique and direct probe of the NSM environment as well insight into the nature of the $r$ process, just as observed gammas from the $^{56}$Ni radioactive decay chain provide a window into supernova nucleosynthesis. In this work, we include the photons from fission processes for the first time in estimates of the MeV gamma-ray signal expected from a NSM event. We consider NSM ejecta compositions with a range of neutron richness and find a dramatic difference in the predicted signal depending on whether or not fissioning nuclei are produced. The difference is most striking at photon energies above $\sim3.5$ MeV and at a relatively late time, several days after the merger event, when the ejecta is optically thin. We estimate that a Galactic NSM could be detectable by a next generation gamma-ray detector such as AMEGO in the MeV range, up to $\sim10^4$ days after the merger, if fissioning nuclei are robustly produced in the event.