Gravitational waves, neutrino emissions, and effects of hyperons in binary neutron star mergers

Numerical simulations for the merger of binary neutron stars are performed in full general relativity incorporating both nucleonic and hyperonic finite-temperature equations of state (EOS) and neutrino cooling. It is found that for the nucleonic and hyperonic EOS, a hyper massive neutron star (HMNS) with a long lifetime $(t_{\rm life}\gtrsim 10 {\rm ms})$ is the outcome for the total mass $\approx 2.7 M_\odot$. For the total mass $\approx 3 M_\odot$, a long-lived (short-lived with $t_{\rm life}\approx 3 {\rm ms}$) HMNS is the outcome for the nucleonic (hyperonic) EOS. It is shown that the typical total neutrino luminosity of the HMNS is $\sim 3$ – $6 \times 10^{53} {\rm erg /s}$ and the effective amplitude of gravitational waves from the HMNS is 1 – $4\times 10^{-22}$ at $f\approx 2$ – $3.2 {\rm kHz}$ for a source of distance of 100 Mpc. During the HMNS phase, characteristic frequencies of gravitational waves shift to a higher frequency for the hyperonic EOS in contrast to the nucleonic EOS in which they remain constant approximately. Our finding suggests that the effects of hyperons are well imprinted in gravitational wave and its detection will give us a potential opportunity to explore the composition of the neutron star matter. We present the neutrino luminosity curve when a black hole is formed as well.

💡 Research Summary

This paper presents fully general‑relativistic numerical simulations of binary neutron‑star (BNS) mergers that incorporate finite‑temperature equations of state (EOS) with and without Λ‑hyperons, together with a neutrino‑leakage cooling scheme. Two EOS families are employed: a purely nucleonic EOS (based on the SLy model with thermal extensions) and a hyperonic EOS that adds Λ‑hyperons to the same nucleonic sector, thereby softening the pressure at supranuclear densities and reducing the maximum non‑rotating mass by roughly ten percent. The authors explore two total binary masses, ≈2.7 M⊙ and ≈3 M⊙, each simulated with both EOSs.

For the lower mass case (≈2.7 M⊙), both EOSs produce a hyper‑massive neutron star (HMNS) that survives for more than 10 ms, supported by rapid differential rotation and thermal pressure. The neutrino luminosity during this phase is high, Lν≈3–6 × 10⁵³ erg s⁻¹, dominated by electron‑type neutrinos (νe, ν̄e) with a ≈20 % contribution from heavy‑flavor neutrinos (νx). The gravitational‑wave (GW) signal exhibits a strong peak at 2–2.5 kHz, and the characteristic frequency remains essentially constant throughout the HMNS lifetime.

In the higher‑mass case (≈3 M⊙), the EOS dependence becomes dramatic. The nucleonic EOS still yields a long‑lived HMNS (t ≳ 20 ms), whereas the hyperonic EOS leads to a rapid collapse to a black hole after only ≈3 ms. This difference is traced to the additional softening caused by hyperons, which accelerates the contraction of the remnant. The GW spectrum for the hyperonic case shows a clear upward drift of the dominant frequency: starting near 2.5 kHz it climbs to 3.2 kHz as the HMNS shrinks, reflecting the increasing average density. By contrast, the nucleonic case maintains a nearly stationary frequency. At a fiducial distance of 100 Mpc, the effective GW strain is 1–4 × 10⁻²² in the 2–3.2 kHz band, a range accessible to planned third‑generation detectors such as the Einstein Telescope and Cosmic Explorer.

The study also reports the neutrino light curve associated with black‑hole formation. When the HMNS collapses, the total neutrino luminosity drops sharply, but a brief spike in heavy‑flavor neutrinos occurs due to the sudden heating of the surrounding material. This feature could serve as a multimessenger marker of the exact collapse time if detected by next‑generation neutrino observatories (e.g., Hyper‑Kamiokande, IceCube‑Gen2).

Overall, the work demonstrates that hyperons leave a distinct imprint on both the GW frequency evolution and the neutrino emission during the post‑merger phase. The upward frequency shift in the hyperonic scenario provides a robust, observable signature that can be used to discriminate between stiff nucleonic and softer hyperonic EOSs. Consequently, combined GW‑neutrino observations of future BNS mergers will offer a powerful avenue to probe the composition of matter at several times nuclear saturation density, potentially confirming or ruling out the presence of hyperons in neutron‑star interiors.