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

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.

9 pages, 5 figures, v2 matches version to appear in ApJL

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• Nucleosynthesis and observation of the heaviest elements
• Actinide opacities for modeling the spectra and light curves of kilonovae
• Astromers in the radioactive decay of r-process nuclei
• r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae
• Gamma-Ray Emission Produced by $r$-process Elements from Neutron Star Mergers
• Gamma-ray Diagnostics of r-process Nucleosynthesis in the Remnants of Galactic Binary Neutron-Star Mergers
• Isochronic evolution and the radioactive decay of r-process nuclei
• Proposed Lunar Measurements of $r$-Process Radioisotopes to Distinguish Origin of Deep-sea 244Pu
• Correlations between energy and $\gamma$-ray emission in ${}^{239}\mathrm{Pu}(n,\mathrm{f})$
• Let there be neutrons! Hadronic photoproduction from a large flux of high-energy photons
• Meaurement of spin vs. TKE of ${}^{144}$Ba produced in spontaneous fission of ${}^{252}$Cf
• Impact of nuclear mass models on $r$-process nucleosynthesis and heavy element abundances in $r$-process enhanced metal-poor stars
• Multi-messengers from the radioactive decay of $r$-process nuclei
• How Do Nuclear Isomers Influence the Gamma-Ray Bursts in Binary Neutron Star Mergers?
• Nuclear data activities for medium mass and heavy nuclei at Los Alamos