What fuels the powerful engine of neutron star mergers?

Computer simulation reveals the dynamo that generates large-scale magnetic fields in merging neutron stars and that may result in high-energetic gamma-ray outbursts

February 15, 2024

Merging and colliding neutron stars produce powerful kilonova explosions and gamma-ray bursts. Scientists have long suspected that a large and ultra-strong magnetic field is the engine behind these high-energy phenomena. However, the process that generates this magnetic field has been a mystery until now. Researchers at the Max Planck Institute for Gravitational Physics and at the universities in Kyoto and Toho have revealed the underlying mechanism by performing a super-high resolution computer simulation taking into account all fundamental physics. The researchers showed that ultra-strongly magnetized neutron stars, also known as magnetars, cause very bright kilonova explosions. Telescopic observations could test this prediction in the future.

Neutron stars are compact remnants of supernova explosions and consist of extremely dense matter. They are about 20 kilometers across and have up to twice the mass of our Sun, or almost 700,000 times the mass of our Earth. On August 17, 2017, astronomers observed for the first time gravitational waves, light, and gamma rays from the merger of two neutron stars. This event marked the beginning of a new kind of multi-messenger astronomy, combining gravitational-wave and electromagnetic observations.

Observations of the gravitational waves and the gamma-ray burst emitted during the merger revealed that binary neutron star mergers are the origin of at least a part of short-hard gamma-ray bursts and the heavy elements. “Only by performing a numerical simulation that takes into account all the fundamental physical effects in binary neutron star mergers will we fully understand the complete process and its underlying mechanisms,” explains Masaru Shibata, director of the Computational Relativistic Astrophysics department at the Max Planck Institute for Gravitational Physics in Potsdam. “That’s why we ran a merger simulation that took into account all the implications of Einstein’s theory of relativity and all other fundamental physics, with a spatial resolution more than ten times higher than any previous simulation, and the highest ever.”

New computer simulation reveals the dynamo that generates large-scale magnetic fields in merging neutron stars.

What fuels the powerful engine of neutron star mergers?

New computer simulation reveals the dynamo that generates large-scale magnetic fields in merging neutron stars.

As in the Sun so in the neutron star

High-energy phenomena associated with neutron-star mergers such as kilonova explosions and gamma-ray bursts are most likely driven by magnetohydrodynamics—the interplay between magnetic fields and fluids. This implies that a binary neutron star merger remnant must generate a strong, large-scale magnetic field via a dynamo mechanism.

“For the first time, we could pin down the physical mechanism that generates a large-scale magnetic field from smaller ones in binary neutron star mergers,” says Kenta Kiuchi, group leader in the Computational Relativistic Astrophysics department. “Part of this mechanism is the same that drives our Sun’s magnetic field. In a neutron star merger, the large-scale magnetic field emerges because of instabilities and vortices at the surface where the two neutron stars slam into one another.”

There are two phases of magnetic field amplifications: In a first phase, the Kelvin-Helmholtz instability rapidly amplifies the energy in the magnetic field by a factor of several thousand within a few milliseconds after the merger. “However, this amplified magnetic field still is a small-scale field,” explains Alexis Raboul-Salze, postdoctoral researcher in the Computational Relativistic Astrophysics department. “But after a few milliseconds, there is a second phase of magnetic field amplification due to another instability, the magnetorotational instability. This instability further amplifies the small-scale field and acts as a dynamo on the large-scale field – the same mechanism as in the Sun.”

The resulting highly magnetized massive neutron star born in the collision is hypothetically proposed as a magnetar. About 40 milliseconds after the merger the magnetic fields drives a strong particle wind at relativistic speeds from the poles of the magnetar. This wind forms a jet, which is related to the observed high-energy phenomena. The research group shows that this hypothesis is feasible for the first time. 

“Our simulation suggests that the magnetar engine generates very bright kilonova explosions. We can test our prediction by multi-messenger observations in the near future,” concludes Masaru Shibata.  

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