Impact of Magnetic Field Topology on Electromagnetic and Gravitational Waves from Binary Neutron Star Merger Remnants

TL;DR

This study uses high-resolution GRMHD simulations to explore how interior magnetic-field topology in binary neutron star mergers affects post-merger dynamics, gravitational waves, and electromagnetic counterparts. By comparing four topologies (P, SP, T_0.95, T_0.5) across two EoSs (SLy, WFF1) and a low-mass SLy case, it demonstrates that magnetic topology can shift the dominant post-merger GW frequency f2 and even induce a nonlinearly coupled m=0–m=2 mode, with such shifts potentially detectable by third-generation detectors up to ~50 Mpc. The work also shows that purely poloidal fields maximize jet viability and magnetically-powered outflows consistent with sGRBs, while ejecta masses and kilonova signatures remain broadly robust across topologies. Overall, the findings highlight the critical role of interior magnetic topologies in shaping multimessenger signals from BNS remnants and point to future neutrino- and microphysics-inclusive simulations for more complete predictions.

Abstract

We perform general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers with four distinct magnetic field topologies: (i) a dipole pulsar-like configuration, (ii) a mixed linear superposition of poloidal and toroidal components inside the star, and (iii-iv) two topologies featuring a smooth transition from a confined mixed core to a pulsar-like structure at radii $0.95\,R_{\rm NS}$ and $0.5\,R_{\rm NS}$, with $R_{\rm NS}$ the radius of the star. The latter topologies are explored in BNS merger studies for the first time. We evolve systems with two equations of state (EoS), SLy and WFF1, with ADM masses 2.7 and 2.6, respectively, and include an additional lower-mass SLy binary to probe the behavior of long-lived remnants. We perform an extensive analysis of the emission properties of the systems, both electromagnetic and gravitational waves, and of the properties of the remnants, namely their frequency modes, density eigenfunctions, rotation, temperature, and convective stability. We report three key results: (1) for the first time, we assess the convective stability of magnetized remnants, extending previous unmagnetized analyses; (2) we identify a clear secondary peak in the gravitational-wave spectrum of pulsar-like configurations, consistent with the nonlinear coupling of the $m=0$ and $m=2$ modes, which is absent in other topologies; and (3) the magnetic field topology strongly influences the gravitational wave emission properties to the extent that nearby ($<50\,{\rm Mpc}$) events could allow one to observationally distinguish between different field structures with future gravitational-wave detectors. Across all models, we obtain luminosities compatible with short gamma-ray bursts (sGRBs), with purely poloidal configurations being the most efficient in driving possible relativistic jets.

Figures (18)

• Figure 1: Volume renderings of the rest-mass density, normalized to its initial maximum value (logarithmic scale), at the moment of magnetic field insertion (left) and after the BNS remnant reaches quasiequilibrium (right). Top panel shows the SLy BNS with a pure poloidal (pulsar-like) magnetic field, while the bottom panel depicts the WFF1 BNS with a mixed magnetic field inside the star ($r_1=0.95\,R_{\rm NS}$, see Sec. \ref{['sec:magnetic-fields']}), smoothly transitioning to an external dipole field. Insets highlight the initial magnetic field configuration inside the star (left), and the helical magnetic structure after an incipient jet is launched from the pole of the the binary's remnant (right top panel BH remnant and right bottom NS remnant). White lines represent magnetic field lines, arrows indicate plasma velocities, and the black sphere marks the BH apparent horizon. Here, $M=2.7M_\odot$ for the top panels and $M=2.6M_\odot$ for the bottom panels, with $M_\odot=4.9\times10^{-3}\,{\rm ms} = 1.4\,{\rm km}$.
• Figure 2: Top panel: Pseudocolor plot of the quality factor $Q_{\rm MRI}=\lambda_{\rm MRI}/dx$ on the equatorial (xy) plane for the remnant of the SLy BNS at $\sim 3\,{\rm ms}$ after merger with: i) a pure initial poloidal field (left); a pulsar-like + mixed magnetic field (middle); and an interior mixed field transitioning smoothly to an external pulsar-like magnetic field at $0.95 R_{\rm NS}$ (right). Bottom panel: Rest-mass density normalized to its initial maximum value (logarithmic scale) on a meridional (xz) slice, along with the $\lambda_{\rm MRI}$ (white line) for the same cases as in the top panels. For the most part the $\lambda_{\rm MRI}$ fits within the remnant.
• Figure 3: Average azimuthal angular velocity profiles of the binary remnant that undergo collapse (see Table \ref{['tab:results']}) at $\sim 2\, {\rm ms}$ following merger (faded lines) and at $1\,{\rm ms}$ before BH formation (bold lines). Arrows marks the rest mass density contour at $\rho_0=10^{-3} \rho_{0,\rm max}(t=0)$, which gives a rough measure of the size of the remnant.
• Figure 4: Evolution of the magnetic energy $\mathcal{M}$ as computed via Eq. \ref{['eq:mag-energy']} for cases listed in Table \ref{['table: initial data']} endowed with the magnetic field configurations in Sec. \ref{['sec:magnetic-fields']}. The cyan circles mark the BH formation time. The inset highlights a slight decrease in $\mathcal{M}$ just after BH formation in the SLy_SP case. However, its magnetic energy continues to increase beyond this point.
• Figure 5: Average azimuthal angular velocity profiles of the stable binary remnants (see Table \ref{['tab:results']}) in at $\sim 2\,{\rm ms}$ following merger (faded lines) and near to the termination of our simulations (bold lines). Arrows marks the rest mass density contour at $\rho_0=10^{-3} \rho_{0,\rm max}(t=0)$, which gives a rough measure of the size of the remnant.