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

When binary neutron stars merge, they produce remarkable signals that help astrophysics understand their composition and magnetized environments. The strong magnetic fields of neutron stars orbiting each other interact vividly. Astronomers are excited about possible electromagnetic signals before the merger – to foreshadow a neutron star collision before studying them in detail. The so-called Kelvin-Helmholtz instability at the interaction layer between the companions is one way to produce such signals. Our simulations show: 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 has even more complex dynamics, like flares and coronal mass ejections.

Research summary

Neutron stars in a simple computational setup with aligned magnetic fields and separated several stellar radii can dissipate up to 40% of their spin-down luminosity in an ultra-magnetized Kelvin-Helmholtz layer between the companions.
For systems in which one companion has an inclined magnetic axis, Kelvin-Helmholtz episodes appear periodically. In such cases, magnetic field lines can connect the two companions and drive additional dissipation through reconnection.
Dynamics in the interaction layer (Kelvin-Helmholtz and flaring) can drive 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).
Especially systems with inclinations have rich dynamics, development of electric zones, and increased dissipation.

Visualizing science

Schematic overview of the conducted binary neutron star simulations (mesh structure, top left panel). Two dipole magnetospheres rotate next to each other (neglected effects of orbital dynamics for simplicity). In the inner interaction layer, typical Kelvin-Helmholtz dynamics develop (vortices in the charge density, top right panel). The field line velocity (bottom right panel) varies close to the interaction layer, and magnetospheric mixing occurs. Magnetic field lines slow down by the Kelvin-Helmholtz drag and are released in a slingshot-like motion (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 binary 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 hint at the occurrence of fast magnetosonic waves. Regions of strong electric fields are shown in red-to-yellow colors. Spherical and outwards propagating structures with strong electric fields may generate shocks in the binary wind, possible sites for FRB generation.

Strength of the electric field relative to the magnetic field for two interacting magnetospheres with strong inclination. Concentric structures hint at the occurrence of fast magnetosonic waves. Regions of strong electric fields are shown in red-to-yellow colors. Spherical and outwards propagating structures with strong electric fields may generate shocks in the binary wind, possible sites for FRB generation.

Collaborative results

Mahlmann, J. F., & Beloborodov, A. M. (in preparation).