MeV gamma rays from fission: a distinct signature of actinide production in neutron star mergers

X. Wang, N. Vassh, T. M. Sprouse, M. Mumpower, R. Vogt, J. Randrup, R. Surman

In Press ApJL 903 1 (2020)

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.

LA-UR-20-25892

@article{Wang2020MeV,
  title = {{MeV} Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers},
  volume = {903},
  issn = {2041-8213},
  doi = {10.3847/2041-8213/abbe18},
  number = {1},
  journal = {The Astrophysical Journal Letters},
  publisher = {American Astronomical Society},
  author = {Wang 王夕露), Xilu and Vassh, Nicole and Sprouse, Trevor and Mumpower, Matthew and Vogt, Ramona and Randrup, Jorgen and Surman, Rebecca},
  year = {2020},
  month = {Oct},
  pages = {L3},
}

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