General-relativistic radiation magnetohydrodynamics simulations of binary neutron star mergers: The influence of spin on the multi-messenger picture

TL;DR

This study addresses how neutron-star spin influences the multi-messenger signals of binary neutron star mergers by performing high-resolution GRRMHD simulations with BAM, including M1 neutrino transport and an ABHT(QMC-RMF3) EOS. It contrasts non-spinning and spin-aligned equal-mass binaries, finding that spin delays merger (orbital hang-up) and yields a more extended remnant with a larger disk, while the non-spinning case produces more dynamical ejecta at higher velocities. Magnetic amplification proceeds via Kelvin–Helmholtz instabilities early on and is enhanced later by winding and MRI, with the non-spinning case achieving stronger late-time fields; ejecta composition and Ye variation drive distinct nucleosynthesis yields. Post-processing with WinNet, possis, and pyblastafterglow connects the dynamics to observables, predicting brighter kilonova emission for non-spinning binaries and providing GW post-merger spectra and afterglow signatures, while acknowledging missing physics such as muons and resistive MHD. These results advance the interpretation of future multi-messenger detections and guide expectations for EM signals given different spin configurations, within current numerical limits.

Abstract

The rich phenomenology of binary neutron star mergers offers a unique opportunity to test general relativity, investigate matter at supranuclear densities, and learn more about the origin of heavy elements. As multi-messenger sources, they emit both gravitational waves and electromagnetic radiation across several frequency bands. The interpretation of these signals relies heavily on accurate numerical-relativity simulations that incorporate the relevant microphysical processes. Using the latest updates of the BAM code, we perform general-relativistic radiation magnetohydrodynamic simulations of binary neutron star mergers with two different spin configurations. We adopt a state-of-the-art equation of state based on relativistic mean-field theory developed for dense matter in neutron star mergers. To capture both dynamical ejecta and secular outflows from magnetic and neutrino-driven winds, we evolve the systems up to $\sim 100\ \rm ms$ after the merger at considerably high resolution with a grid spacing of $Δx \approx 93\ \rm m$ across the neutron stars. Our results show that the non-spinning configuration undergoes a more violent merger, producing more ejecta with lower electron fraction and higher velocities, while the spinning configuration forms a larger disk due to its higher angular momentum. Although the initial magnetic field amplification within $\lesssim 10\ \rm ms$ after merger is similar in both systems, the non-spinning system reaches stronger magnetic fields and higher energies at later times. For a detailed view of the multi-messenger observables, we extract the gravitational-wave signal and compute nucleosynthesis yields, the expected kilonova and afterglow light curves from our ejecta profiles.

Figures (25)

• Figure 1: BNS merger phenomena for different timescales, ranging from seconds to milliseconds before the merger and from milliseconds to years after the merger. As simplified overview, we illustrate the physical phenomena together with the associated observable multi-messenger signals, consisting of GWs and EM signatures: inspiral and merger of the two neutron stars, ejection of lanthanide-rich (in red) and lanthanide-poor (in blue) material, formation of a black hole remnant with accretion disk, launch of a relativistic jet (in purple), formation of heavy elements via $r$-process, kilonova emission (in yellow), and non-thermal afterglows of the GRB (in purple) and the kilonova (in darkred).
• Figure 2: 3D-Snapshots of the high-resolution simulation without spin. Each panel shows a rendering of the rest-mass density in purple to orange scales and magnetic field lines in blue. For visualization, the rest-mass density is sliced along the $x$-$z$ plane. Additionally, insets sliced along the $x$-$y$ plane are shown in the upper left corner of each panel. The snapshots are extracted from refinement level $l=3$ and represent different stages of the merger and post-merger phase, visualizing the evolution and winding of the magnetic field lines.
• Figure 3: Evolution of maximum rest-mass density $\rho_{\rm max}$ (top panel) and maximum temperature $T_{\rm max}$ (bottom panel). Results of the R2 and R1 simulations are shown in solid and dashed lines, respectively, for the non-spinning (green) and spinning (purple) configurations extracted from refinement level $l=6$. For a smoother visualization, data is presented with a moving-average window of width $0.2\ \rm ms$.
• Figure 4: Evolution of maximum magnetic field strength $B_{\rm max}$ (top panel) and magnetic energy $E_{\rm mag}$ (bottom panel). Results of the R2 and R1 simulations are shown in solid and dashed lines, respectively, for the non-spinning (green) and spinning (purple) configurations extracted from refinement level $l=6$ for $B_{\rm max}$ and $l=1$ for $E_{\rm mag}$.
• Figure 5: Shear layer of the non-spinning (upper panels) and spinning (lower panels) BNS systems at merger time for the simulations with R2 resolution. The snapshots show the magnetic pressure $P_{\rm mag} = 0.5 b^2$ in the $x$-$y$ plane at $\{0,1,2.5,5,10 \}\ \rm ms$ after the merger at refinement level $l=5$. The yellow lines are contours of the rest-mass density at $\rho = \{ 10^{10},10^{12},10^{14}\} \ {\rm g/cm^3}$, respectively as dotted, dashed, and solid lines.