The imprint of individual neutron star spins on ejecta, r-process nucleosynthesis, and kilonovae in binary neutron star mergers
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.
Related
- New Constraints on r-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-phase Spectra
- New Constraints on r-process Nucleosynthesis in Neutron Star Mergers from GW170817 Late-Phase Spectra
- The MARTINI platform II. Tracing r-process nucleosynthesis in neutron star mergers with long-lived remnants
- Cluster emission and its impact on the r-process nucleosynthesis
- Magnetic Eruption and Nucleosynthesis in GRνMHD Simulations of Spinning Neutron Star Mergers
BibTeX
@misc{karakas2026imprint,
title = {{The imprint of individual neutron star spins on ejecta, $r$-process nucleosynthesis, and kilonovae in binary neutron star mergers}},
author = {Beyhan Karakas and Rahime Matur and David Radice and Roland Haas and Maximilian Ruffert},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2608.22058},
eprint = {2608.22058},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.22058},
}
RIS
TY - UNPB
TI - The imprint of individual neutron star spins on ejecta, r-process nucleosynthesis, and kilonovae in binary neutron star mergers
AU - Beyhan Karakas
AU - Rahime Matur
AU - David Radice
AU - Roland Haas
AU - Maximilian Ruffert
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2608.22058
UR - https://arxiv.org/abs/2608.22058
AB - To leading order, the gravitational-wave (GW) signal from binary neutron stars depends on the individual spins, χ_1 and χ_2, only through the effective spin parameter χ_eff. 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 χ_eff. 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 M_tot=2.55 M_⊙ at fixed χ_eff=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 χ_eff=0. The kilonova retains the individual-spin imprint, with peak brightness differences reaching ∼0.9 mag. At 40 Mpc, all three fixed χ_eff=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.
ER -