How ultra-magnetized Kelvin-Helmholtz drag impacts precursor emission in neutron star binaries

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When binary neutron stars merge, they produce remarkable signals that help astrophysicists understand their composition and magnetized environments. The strong magnetic fields of two neutron stars orbiting each other interact vigorously. Astronomers hope to detect electromagnetic signals before the merger, which would announce a neutron star collision before it happens and allow it to be studied in detail. The so-called Kelvin-Helmholtz instability in the interaction layer between the companions is one way to produce such signals. Our simulations show that magnetospheric mixing in Kelvin-Helmholtz vortices releases significant electromagnetic energy. Waves produced during the binary interaction can become very strong and drive powerful shocks and fast radio bursts. If the neutron stars have inclined magnetic fields, their environment shows even more complex dynamics, such as flares and coronal mass ejections.

Research summary

  • In a simple computational setup with aligned magnetic fields and a separation of several stellar radii, neutron stars can dissipate up to 40% of their spin-down luminosity in an ultra-magnetized Kelvin-Helmholtz layer between the companions.
  • In systems where one companion has an inclined magnetic axis, Kelvin-Helmholtz episodes appear periodically. Magnetic field lines can then connect the two companions and drive additional dissipation through reconnection.
  • Kelvin-Helmholtz dynamics and flaring in the interaction layer drive a ubiquitous injection of fast magnetosonic waves. These waves propagate across magnetic field lines and can develop regions in which their electric field becomes as strong as the magnetic field. These electric zones can heat plasma or accelerate particles in shocks. Shocks in the magnetized binary wind can become a source of fast radio bursts (FRBs).
  • Systems with inclined magnetic fields show particularly rich dynamics, more electric zones, and increased dissipation.

Visualizing science

Schematic overview of the binary neutron star simulations, with the mesh structure in the top left panel. Two dipole magnetospheres rotate next to each other, and orbital motion is neglected for simplicity. Typical Kelvin-Helmholtz dynamics develop in the inner interaction layer, visible as vortices in the charge density in the top right panel. The field line velocity in the bottom right panel varies close to the interaction layer, where magnetospheric mixing occurs. Magnetic field lines are slowed down by the Kelvin-Helmholtz drag and released in a slingshot-like motion, as shown in the bottom left panel.
Magnetic-field-aligned currents in a binary system without inclinations. The magnetospheric current sheets warp around each other in the interaction zone between the two magnetospheres. Field lines in the inner magnetosphere can be dragged along the interaction layer and inject compressive waves.
Strength of the electric field relative to the magnetic field for two interacting magnetospheres without inclinations. Concentric structures indicate fast magnetosonic waves. Red-to-yellow colors mark regions of strong electric fields. Spherical, outward-propagating structures with strong electric fields may generate shocks in the binary wind, which are possible sites of FRB generation.
Strength of the electric field relative to the magnetic field for two interacting magnetospheres with strong inclination. Concentric structures indicate fast magnetosonic waves. Red-to-yellow colors mark regions of strong electric fields. Spherical, outward-propagating structures with strong electric fields may generate shocks in the binary wind, which are possible sites of FRB generation.

Collaborative results

Mahlmann, J. F., & Beloborodov, A. M. Electrodynamics and dissipation in the binary magnetosphere of pre-merger neutron stars, arXiv:2412.16280