Neutron stars, the dense remnants left behind by supernova explosions, are among the universe's most fascinating phenomena. With diameters of about 20 kilometers and masses up to twice that of the Sun, their incredible density makes them key subjects in the study of extreme physics. The observation of gravitational waves, light, and gamma rays from a binary neutron star merger on August 17, 2017, marked a significant milestone in astrophysics, heralding the era of multi-messenger astronomy. This integrated approach, which combines gravitational-wave and electromagnetic observations, offers a more comprehensive understanding of cosmic events.

The collaborative research effort has now provided insights into the generation of the strong magnetic fields responsible for the energetic phenomena associated with neutron star mergers. "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," states Masaru Shibata, director of the Computational Relativistic Astrophysics department at the Max Planck Institute. The simulation conducted by the researchers boasts a spatial resolution more than ten times higher than any previous attempt, making it the highest ever in this field of study.

The study reveals that the process of magnetic field generation in neutron star mergers is akin to that observed in our Sun. Through the interplay of magnetohydrodynamics-the interaction between magnetic fields and fluids-a large-scale magnetic field emerges from smaller ones due to instabilities and vortices at the collision interface of the neutron stars. Kenta Kiuchi, the group leader in the Computational Relativistic Astrophysics department, highlights the significance of these findings, noting the parallel between the mechanisms driving the Sun's magnetic field and those observed in neutron star mergers.

The research identifies two phases of magnetic field amplification. Initially, the Kelvin-Helmholtz instability rapidly enhances the magnetic field's energy within milliseconds following the merger. Subsequently, the magnetorotational instability further amplifies this field, acting as a dynamo that generates a large-scale magnetic field. This process, previously theorized, has been confirmed for the first time through this simulation.

The resultant magnetar, a highly magnetized massive neutron star born from the collision, is believed to drive strong particle winds at relativistic speeds from its poles, forming a jet that contributes to the observed kilonova explosions and gamma-ray bursts. "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 Shibata, expressing optimism about validating these findings through upcoming astronomical observations.

This research not only enhances our understanding of the fundamental processes driving some of the universe's most energetic events but also underscores the importance of high-resolution simulations in unlocking the mysteries of cosmic phenomena. As we stand on the cusp of new discoveries in multi-messenger astronomy, the insights gained from this study pave the way for deeper exploration into the universe's most extreme events.

Research Report:A large-scale magnetic field produced by a solar-like dynamo in binary neutron star mergers

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Researchers at the Max Planck Institute for Gravitational Physics
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