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

Journal Article · · The Astrophysical Journal. Letters (Online)
 [1];  [2];  [3];  [4];  [5];  [6];  [2]
  1. Univ. of California, Berkeley, CA (United States); Univ. of Notre Dame, IN (United States)
  2. Univ. of Notre Dame, IN (United States)
  3. Univ. of Notre Dame, IN (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  4. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  5. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  6. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
+ Show Author Affiliations
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 ~104 days after the merger, if fissioning nuclei are robustly produced in the event.
Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA)
Contributing Organization:
FIRE Collaboration; N3AS Collaboration
Grant/Contract Number:
AC52-07NA27344
OSTI ID:
1736327
Alternate ID(s):
OSTI ID: 23016170
Report Number(s):
LLNL-JRNL-813431; 1021444
Journal Information:
The Astrophysical Journal. Letters (Online), Journal Name: The Astrophysical Journal. Letters (Online) Journal Issue: 1 Vol. 903; ISSN 2041-8213
Publisher:
Institute of Physics (IOP)Copyright Statement
Country of Publication:
United States
Language:
English

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79 ASTRONOMY AND ASTROPHYSICS