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

The imprint of individual neutron star spins on ejecta, r -process nucleosynthesis, and kilonovae in binary neutron star mergers

  • Beyhan Karakaş1
  • Rahime Matur2
  • David Radice3,4,5
  • Roland Haas6,7,8
  • Maximilian Ruffert9
9 affiliations
  1. 1[email protected]
  2. 2Mathematical Sciences and STAG Research Centre, University of Southampton , Southampton SO17 1BJ ,
  3. 3Institute for Gravitation and the Cosmos, The Pennsylvania State University , University Park, PA 16802 ,
  4. 4The Pennsylvania State University Department of Physics, , University Park, PA 16802 ,
  5. 5The Pennsylvania State University Department of Astronomy, and Astrophysics, , University Park, PA 16802 ,
  6. 6University of British Columbia Department of Physics and Astronomy, , Vancouver, British Columbia ,
  7. 7National Center for Supercomputing applications, University of Illinois , Urbana, Illinois ,
  8. 8University of Illinois Department of Physics, , Urbana, Illinois ,
  9. 9School of Mathematics and Maxwell Institute, University of Edinburgh , Edinburgh EH9 3FD ,
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Abstract

To leading order, the gravitational-wave (GW) signal from binary neutron stars depends on the individual spins, χ1 and χ2, only through the effective spin parameter χeff. We present the first systematic investigation of individual-spin effects on ejecta, r-process nucleosynthesis, and kilonova emission, including comparisons at fixed total mass, mass ratio and χeff. We use numerical relativity ejecta from three total mass regimes with the finite-temperature, composition-dependent SFHo equation of state and neutrino emission and absorption. For Mtot = 2.55 M⊙ at fixed χeff = 0, individual spins change the dynamical ejecta mass by a factor of ∼45, while the absolute A ≥ 140 yield spans more than two orders of magnitude and the lanthanide to light r-process mass ratio increases from ∼2 to ∼70. Prompt-collapse 4.10 M⊙ models show heavy-element yield differences exceeding four orders of magnitude at χeff = 0. The kilonova retains the individual-spin imprint, with peak brightness differences reaching ∼0.9 mag. At 40 Mpc, all three fixed χeff = 0 pairs remain above adopted depths at common epochs for all viewing angles, with same-epoch colour differences reaching ∼1.5 mag. The colour imprint persists when the simulation-derived secular ejecta are replaced by the same parametric disc outflow, indicating that disc mass differences are not the primary driver and that dynamical ejecta make an important contribution. Neutrino absorption systematically brightens the kilonova and shifts peak-associated colours blueward. These EM signatures can break the degeneracy between individual spins in the GW signal.

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Cite

BibTeXRIS
BibTeX
@article{karakas2026imprint,
  title = {{The imprint of individual neutron star spins on ejecta, r -process nucleosynthesis, and kilonovae in binary neutron star mergers}},
  author = {Beyhan Karakaş and Rahime Matur and David Radice and Roland Haas and Maximilian Ruffert},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = nov,
  doi = {10.1093/mnras/stag2097},
  url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2097/9011262},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - The imprint of individual neutron star spins on ejecta, r -process nucleosynthesis, and kilonovae in binary neutron star mergers
AU  - Beyhan Karakaş
AU  - Rahime Matur
AU  - David Radice
AU  - Roland Haas
AU  - Maximilian Ruffert
PY  - 2026
DA  - 2026/11/03
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag2097
UR  - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2097/9011262
AB  - To leading order, the gravitational-wave (GW) signal from binary neutron stars depends on the individual spins, χ1 and χ2, only through the effective spin parameter χeff. We present the first systematic investigation of individual-spin effects on ejecta, r-process nucleosynthesis, and kilonova emission, including comparisons at fixed total mass, mass ratio and χeff. We use numerical relativity ejecta from three total mass regimes with the finite-temperature, composition-dependent SFHo equation of state and neutrino emission and absorption. For Mtot = 2.55 M⊙ at fixed χeff = 0, individual spins change the dynamical ejecta mass by a factor of ∼45, while the absolute A ≥ 140 yield spans more than two orders of magnitude and the lanthanide to light r-process mass ratio increases from ∼2 to ∼70. Prompt-collapse 4.10 M⊙ models show heavy-element yield differences exceeding four orders of magnitude at χeff = 0. The kilonova retains the individual-spin imprint, with peak brightness differences reaching ∼0.9 mag. At 40 Mpc, all three fixed χeff = 0 pairs remain above adopted depths at common epochs for all viewing angles, with same-epoch colour differences reaching ∼1.5 mag. The colour imprint persists when the simulation-derived secular ejecta are replaced by the same parametric disc outflow, indicating that disc mass differences are not the primary driver and that dynamical ejecta make an important contribution. Neutrino absorption systematically brightens the kilonova and shifts peak-associated colours blueward. These EM signatures can break the degeneracy between individual spins in the GW signal.
SN  - 0035-8711
SN  - 1365-2966
ER  -