Effect of spin in binary neutron star mergers

TL;DR

This work addresses how spin influences binary neutron star mergers by performing state-of-the-art 3+1 general relativistic hydrodynamics simulations with neutrino transport, using the SFHo microphysical EoS across three total masses ($M_{\,\mathrm{tot}} = 2.55,\ 3.05,\ 4.10\,M_{\odot}$) and diverse spin configurations (aligned, anti-aligned, mixed). The authors systematically track dynamics, remnant structure and spin, ejecta masses and composition, disc masses, gravitational waves (including mode frequencies such as $f_2$ and $f_1$), and neutrino energies/luminosities, revealing that spin exerts strong and orientation-dependent effects on all these channels. A key finding is the transition around $|\chi|\approx0.4$, where spin-orbit and spin-spin interactions compete, altering inspiral timing and remnant properties; highly spinning aligned binaries can produce substantial ejecta ($\sim0.06\,M_{\odot}$) and large discs, while high-mass, misaligned cases tend to form prompt BHs. The study also demonstrates the potential to constrain neutron star spins from electromagnetic counterparts, albeit with degeneracies with the EoS in GW spectra, and to identify the record-setting BH spin ($\chi\approx0.92$) in a BNS merger. These results advance the modeling of high-spin BNS mergers and inform interpretation of future multimessenger observations.

Abstract

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.

Figures (10)

• Figure 1: Gravitational wave strains for aligned (top panel) and anti-aligned (bottom panel) spin models, shown up to $0.1$ ms after the merger from high resolution simulations. The strains are aligned at the merger time (vertical dashed line), with the change in trend with spin distinctly visible in both panels: note differences in merger times.
• Figure 2: The temperature distribution of the remnant and the inner disc region is shown for $M_{\mathrm{tot}}=2.55M_{\odot}$ at $20$ ms after the merger in the $x$-$y$ plane. The purple contour marks a rest-mass density of $\rho=10^{13} \, \mathrm{g cm}^{-3}$, black contours denote $\rho=10^{12}$, $10^{14}$ and $10^{15} \, \mathrm{g cm}^{-3}$. Panels represent the different spin configurations, illustrating the impact of spin on the temperature structure of the remnant.
• Figure 3: The distribution of electron fraction $Y_{e}$ in the remnant and disc is shown at $20$ ms after the merger. Both $Y_{e}$ and the rest-mass density contours are presented on logarithmic scales, with black contours marking densities of $\rho= 10^{6}, \, 10^{7}, \, 10^{8}, \, 10^{9}, \, 10^{10}, \, 10^{11}, \, 10^{12} \, \mathrm{g cm}^{-3}$, and the purple contour representing $\rho = 10^{13} \, \mathrm{g cm}^{-3}$. This figure highlights the impact of spin on the composition.
• Figure 4: Neutrino luminosities and mean energies for electron neutrinos ($\nu_e$), electron anti-neutrinos ($\bar{\nu}_e$), and heavy‐lepton neutrinos ($\nu_x$) for $M_{\mathrm{tot}}=2.55\,M_{\odot}$ models with different spins. Anti-aligned spins yield higher mean energies and enhanced luminosities than the aligned spins due to higher neutrino number flux from more compact and hotter remnants. Main panels share a common y-axis to illustrate overall trends, whereas inset panels use their own scales to enhance visualisation of temporal variations.
• Figure 5: Gravitational wave spectra for mode $(l,m)=(2,1)$ for models with $M_\mathrm{{tot}}=2.55M_{\odot}$ showing their detectability by the Advanced LIGO and the Einstein Telescope (ET). The dotted lines indicate the sensitivity curves of the detectors etd_sensitivityaligo_O4high