Binary neutron star mergers

Phys. Rev. Lett. (accepted) · 2026

Turbulent dynamo action in binary neutron star mergers

In (Gutiérrez et al., 2026), we studied how weak, pulsar-like magnetic fields are amplified during the merger. When the two stars touch, the shear layer between them becomes unstable to the Kelvin-Helmholtz instability, and the vortices it produces drive a fast turbulent small-scale dynamo that fills the remnant with strong, tangled magnetic fields.

(a) Temperature in the orbital plane at merger, with a zoom into the shear layer between the stars and its velocity field. (b) Magnetic field strength at t ≃ 0.25 ms, as Kelvin-Helmholtz vortices develop along the shear layer. (c) The turbulent remnant at t ≃ 1.5 ms, with successive zoom-ins. (d) 3D volume rendering of the magnetic field strength.

Resolving this dynamo requires extremely fine grids. We ran a series of simulations with grid spacings down to a few meters, and found that the magnetic energy grows by about five orders of magnitude within roughly 2 ms of merger at the highest resolutions, where large-scale coherent structures also begin to emerge. This suggests that strong and ordered magnetic fields may be present shortly after merger, with consequences for the electromagnetic and gravitational-wave signals of the remnant.

Growth of the magnetic field strength (top) and magnetic energy (bottom) for grid spacings from 92 m down to 2.9 m.

Phys. Rev. D 114, 063049 · 2026

Magnetic field configurations I: post-merger remnant and disk

In (Gutiérrez et al., 2026), we performed a systematic study of how different initial magnetic field configurations evolve in the post-merger remnant and the surrounding accretion disk. Using high-resolution GRMHD simulations with various initial field geometries, we found that the field is amplified soon after merger by the Kelvin-Helmholtz instability, and identified how the magnetic field topology affects the disk structure and outflows.

Toroidal magnetic field structure in a BNS post-merger remnant.

Phys. Rev. D 114, 063050 · 2026

Magnetic field configurations II: inspiral, merger and ejecta

In the companion paper (Cook et al., 2026), led by William Cook, we followed the same set of magnetic field configurations through the inspiral and merger. We studied how the initial field shapes the gravitational-wave signal, the global amplification of the field, and the properties of the ejected material, and found that the choice of initial field can noticeably affect the phase differences between the dominant and subdominant gravitational-wave modes.

Phys. Rev. D 111, 063031 · 2025

Cocoon shock breakout emission

In (Gutiérrez et al., 2025), I investigated a novel electromagnetic precursor to the main kilonova emission: the cocoon shock breakout signal. I developed a theoretical framework to predict the properties of this early-time emission, which could serve as an important target for follow-up observations of future BNS merger events.

Schematic diagram of the cocoon shock breakout emission from a BNS merger.

Numerical methods

Code development

I also contribute to the development of the next generation of numerical relativity codes used for these simulations:

• Binary neutron star mergers with neutrino transport in GR-Athena++ (Daszuta et al., 2026).
• A comparison of cell- and vertex-centered adaptive meshes for compact binary simulations (Daszuta et al., 2025).
• Finite-solid-angle Boltzmann radiation transport on dynamical spacetimes in AthenaK (Zhu et al., 2026).