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

TL;DR

This paper investigates MeV gamma-ray emissions from neutron star mergers, highlighting the significance of fission processes in the ejecta for detecting actinide production and providing a new observational signature of r-process nucleosynthesis.

Contribution

First inclusion of fission photon contributions in gamma-ray signal estimates from neutron star mergers, revealing distinct signatures based on fissioning nuclei production.

Findings

Fission processes significantly affect gamma-ray signals above 3.5 MeV.

Detectability of gamma rays from a Galactic NSM extends up to ~10^4 days post-merger.

Gamma-ray signals vary notably with the neutron richness of the ejecta.

Abstract

Neutron star mergers (NSMs) are the first verified sites 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 as insight into the nature of the r process, just as observed gammas from the 56Ni 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 an 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 ~3.5 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 ~10^4 days after the merger, if fissioning nuclei are robustly produced in the event.