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Peer-reviewedCC BY 4.0Monthly Notices of the Royal Astronomical Society

Nucleosynthesis in the fast ejecta of a neutron star merger

  • Lukas Schnabel1
  • Stephan Rosswog1,2
  • Moritz Reichert3
  • Friedrich-Karl Thielemann4,5
5 affiliations
  1. 1Hamburg Observatory, Department of Physics, University of Hamburg , Gojenbergsweg 112, D-21029 Hamburg ,
  2. 2The Oskar Klein Centre, Department of Astronomy, AlbaNova, Stockholm University , SE-10691 Stockholm ,
  3. 3Departament d’Astronomia i Astrofísica, Universitat de València , C/Dr Moliner, 50, E-46100 Burjassot (València) ,
  4. 4Physics Department, University of Basel , Klingelbergstrasse 82, CH-4056 Basel ,
  5. 5Theory, GSI Helmholtz Center for Heavy Ion Research , Planckstrasse 1, D-64291 Darmstadt ,
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Abstract

Neutron star mergers are today considered a major production site for rapid neutron capture elements. While the bulk of the matter escapes at fast but non-relativistic velocities (∼ 0.2 c), a small amount of the dynamically ejected mass reaches mildly relativistic velocities (≳ 0.6 c). It has been suggested earlier, that in such ejecta parts neutrons may avoid being captured and that their decay could power an early blue precursor to the main kilonova event. Here we study in detail the nucleosynthesis in such fast ejecta with nuclear network calculations along both parametrized and numerical relativity trajectories. We find that the nucleosynthesis can be divided into three channels, in one of which a substantial amount of free neutrons survives when the main r-process has frozen out. We develop an accurate (semi-)analytical model that indicates under which conditions free neutrons survive. If the mass fraction of the free neutrons exceeds ∼ 0.05, their β --decay dominates the nuclear heating rate between ∼ 100 and ∼ 104 s. This dominance leads to a pronounced kilonova precursor that should for plausible ejecta parameters be visible for ULTRASAT out to ∼ 200 Mpc. Since at low electron fractions free neutrons can survive even for moderate velocities, mergers with large tidal ejecta, such as asymmetric neutron star mergers or favourable neutron star black hole mergers, may produce particularly bright blue precursors to their subsequent kilonovae.

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Cite

BibTeXRIS
BibTeX
@article{schnabel2026nucleosynthesis,
  title = {{Nucleosynthesis in the fast ejecta of a neutron star merger}},
  author = {Lukas Schnabel and Stephan Rosswog and Moritz Reichert and Friedrich-Karl Thielemann},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = oct,
  doi = {10.1093/mnras/stag1828},
  url = {https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1828/8859314},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Nucleosynthesis in the fast ejecta of a neutron star merger
AU  - Lukas Schnabel
AU  - Stephan Rosswog
AU  - Moritz Reichert
AU  - Friedrich-Karl Thielemann
PY  - 2026
DA  - 2026/10/01
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag1828
UR  - https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1828/8859314
AB  - Neutron star mergers are today considered a major production site for rapid neutron capture elements. While the bulk of the matter escapes at fast but non-relativistic velocities (∼ 0.2 c), a small amount of the dynamically ejected mass reaches mildly relativistic velocities (≳ 0.6 c). It has been suggested earlier, that in such ejecta parts neutrons may avoid being captured and that their decay could power an early blue precursor to the main kilonova event. Here we study in detail the nucleosynthesis in such fast ejecta with nuclear network calculations along both parametrized and numerical relativity trajectories. We find that the nucleosynthesis can be divided into three channels, in one of which a substantial amount of free neutrons survives when the main r-process has frozen out. We develop an accurate (semi-)analytical model that indicates under which conditions free neutrons survive. If the mass fraction of the free neutrons exceeds ∼ 0.05, their β ^--decay dominates the nuclear heating rate between ∼ 100 and ∼ 10^4 s. This dominance leads to a pronounced kilonova precursor that should for plausible ejecta parameters be visible for ULTRASAT out to ∼ 200 Mpc. Since at low electron fractions free neutrons can survive even for moderate velocities, mergers with large tidal ejecta, such as asymmetric neutron star mergers or favourable neutron star black hole mergers, may produce particularly bright blue precursors to their subsequent kilonovae.
SN  - 0035-8711
SN  - 1365-2966
ER  -