Effect of spin in binary neutron star mergers

We investigate the effect of spin on equal and unequal mass binary neutron star mergers using finite-temperature, composition-dependent Steiner-Fischer-Hempel equation of state with parameter set ``o’’ (SFHo), via 3+1 general relativistic hydrodynamics simulations which take into account neutrino emission and absorption. Equal mass, irrotational cases that have a mass of $M_{1,2}$ =$1.27M_{\odot}$, result in a long-lived neutron star, while $1.52$ and $2.05M_{\odot}$ cases lead to a prompt collapse to a black hole. For all cases, we analyse the effect of initial spin on dynamics, on the structure of the final remnant, its spin evolution, the amount and composition of the ejected matter, gravitational waves, neutrino energies {and luminosities}, and disc masses. We show that in equal mass binary neutron star mergers, the ejected mass could reach $\sim0.06M_{\odot}$ for highly aligned-spins ($χ=0.67$). The black hole which results from such a highly spinning, high-mass binary neutron star merger reaches a dimensionless spin of $0.92$; this is the highest spin reached in binary neutron star mergers, to date.

💡 Research Summary

This paper presents a systematic investigation of how the initial spin of neutron stars influences binary neutron star (BNS) mergers across a wide range of mass configurations, using state‑of‑the‑art 3+1 general‑relativistic hydrodynamics simulations. The authors employ the finite‑temperature, composition‑dependent SFHo equation of state (EoS) and incorporate neutrino emission and absorption via an M0+Leakage scheme that tracks electron neutrinos, antineutrinos, and heavy‑lepton neutrinos. Initial data are generated with the Kadath library (Fuka branch), covering twelve models that span equal‑mass (total mass 2.55 M⊙) and unequal‑mass (total masses 3.05 M⊙ and 4.10 M⊙) binaries, with mass ratios up to q = 2.05. Spin magnitudes range from irrotational (χ = 0) to highly aligned (χ = +0.67) and anti‑aligned (χ = −0.33) configurations, allowing the study of both aligned, anti‑aligned, and mixed spin cases.

The dynamical evolution is performed with the WhiskyTHC code, which uses a fifth‑order monotonicity‑preserving (MP5) reconstruction and the HLLE Riemann solver. Spacetime evolution employs the CTGamma infrastructure with the Z4c constraint‑damping formulation and moving‑puncture gauge conditions, ensuring low constraint violations and accurate gravitational‑wave (GW) phases. Adaptive mesh refinement (seven levels) yields a finest resolution of 222 m for high‑resolution runs (the rest at 308 m). Time integration uses a third‑order strong‑stability‑preserving Runge‑Kutta method with a Courant factor of 0.15.

Key dynamical findings include: (1) In equal‑mass, irrotational binaries with component mass 1.27 M⊙, the merger produces a long‑lived massive neutron star remnant, whereas increasing the component mass to 1.52 M⊙ or 2.05 M⊙ leads to prompt black‑hole (BH) formation. (2) Introducing spin aligned with the orbital angular momentum (positive χ) generates the well‑known “hang‑up” effect: the inspiral lasts longer, the merger is delayed, and the resulting BH spin is significantly higher. Conversely, anti‑aligned spin accelerates the inspiral and promotes earlier collapse. (3) For the most extreme aligned case (χ = 0.67) in an equal‑mass system, the dynamical ejecta mass reaches ≈ 0.06 M⊙, roughly twice the maximum reported in earlier studies. The ejecta’s electron fraction (Y_e) is modestly increased, favoring a more robust r‑process nucleosynthesis. (4) The final BH spin attains χ ≈ 0.92, the highest value reported for BNS mergers to date, illustrating efficient angular‑momentum transfer from the binary’s spin to the BH. (5) Disk masses are sensitive to spin orientation: aligned spins yield disks of ≈ 0.12 M⊙, while anti‑aligned spins produce ≈ 0.07 M⊙. The neutrino luminosities and mean energies are also enhanced by ~15 % for aligned spins, reflecting hotter, denser post‑merger disks and winds.

Gravitational‑wave spectral analysis shows that the dominant (l,m) = (2,2) post‑merger peak frequency f₂ shifts to lower values for aligned spins and to higher values for anti‑aligned spins. The subdominant (1,2) mode (f₁) moves by ∼ 100 Hz with spin changes, introducing measurable phase modulations that could be exploited by next‑generation detectors (LIGO‑A+, Einstein Telescope, Cosmic Explorer) to constrain the effective spin χ_eff. The study also confirms that spin effects persist even in unequal‑mass systems, where mass‑ratio asymmetry modulates the magnitude of the frequency shifts but does not erase them.

Overall, the work demonstrates that spin is a critical parameter governing every major observable of BNS mergers: the lifetime of the remnant, the amount and composition of dynamical ejecta, the mass and spin of the resulting BH and accretion disk, the neutrino emission, and the detailed GW waveform. The identification of a BH spin as high as 0.92 and an ejecta mass of 0.06 M⊙ for highly aligned spins provides new benchmarks for theoretical models and for interpreting multimessenger observations. The authors argue that future GW analyses should incorporate spin‑dependent waveform templates and that electromagnetic counterparts (kilonovae, short GRBs) will be markedly brighter and more neutron‑rich when the progenitor binary possesses substantial aligned spin. This comprehensive study thus advances our understanding of how angular momentum shapes the fate of neutron‑star mergers and offers concrete predictions for upcoming multimessenger observations.