Turbulent Dynamo Action in Binary Neutron Star Mergers

Abstract

Binary neutron star mergers are expected to generate intense magnetic fields that power relativistic and non-relativistic outflows and shape their multimessenger signatures. These fields likely arise from the turbulent amplification of initially weak magnetic fields during the merger, particularly via the Kelvin-Helmholtz instability at the collisional interface between the stars. While previous studies have shown efficient amplification to magnetar-level strengths, the degree of large-scale coherence of the resulting field remains uncertain. We present general-relativistic, dynamical spacetime, magnetohydrodynamic simulations following the evolution of initially weak, pulsar-like magnetic fields in a binary neutron star merger. We find rapid magnetic field growth at small scales with clear signatures of small-scale turbulent dynamo action. At the highest resolutions, we additionally observe the emergence of coherent magnetic structures on larger scales. Our results imply that strong, ordered magnetic fields may be present immediately after merger, with important implications for the subsequent evolution of the remnant and its observable electromagnetic and gravitational-wave signals.

Figures (10)

• Figure 1: Turbulent magnetic-field amplification in the merger shear layer.(a) Equatorial snapshot of temperature at the time of hand-off; the right subpanel shows a zoom-in of the hot shear layer region with the arrows representing the velocity vector field in the co-rotating frame. The white box on the right subplot shows the box used in the zoom-in simulation. (b) Magnetic field strength at $t{\simeq} 0.25$ ms in the zoom-in simulation. The shear layer has rotated approximately $90^\circ$ and vortices of different sizes have already formed along it due to the development of the KHI; in these vortices, the magnetic field is amplified. (c) Magnetic field strength at $t{\simeq} 1.5 ~{\rm ms}$. The vortices have merged, and turbulence is fully developed in a thick rotating layer. Note that the upper limit for the magnetic field colormap is higher than in (b). (d) Three-dimensional rendering of the magnetic field strength at $t\approx 0.6~{\rm ms}$, showing the global structure of the field.
• Figure 2: Magnetic field amplification in the early post-merger phase.Upper panel: 99th mass-weighted percentile of the magnetic field strength (thick lines with triangular markers), meaning that only 1% of the mass of the fluid mass attains larger values, together with the maximum field strength (dashed lines). Lower panel: Time evolution of the total magnetic energy within the simulation domain, normalized to its initial value.
• Figure 3: Kinetic energy power spectra at selected times for the six simulations. The short black dashed line shows the Kolmogorov scaling $\sim\!k^{-5/3}$; times and numerical resolutions are indicated in the in-panel labels.
• Figure 4: Magnetic energy spectra at selected times for the six simulations. The short black dashed line shows the Kazantsev scaling $\sim\!k^{3/2}$; times and numerical resolutions are indicated in the in-panel labels.
• Figure 5: Equatorial slices of representative variables from the global simulation. From top to bottom: Rest-mass density ($\rho$), temperature ($T$), and magnetic field strength($|{\bf B}|$). From left to right: $t\simeq 0, 16, 19$ ms.