Fugaku simulation of a neutron star merger yields magnetar-strength fields within milliseconds
A simulation on Japan's Fugaku supercomputer of two merging neutron stars of 1.35 solar masses each produced magnetar-strength fields. This is a calculation, not an observation.
Researchers simulated the merger of two neutron stars on Japan's FUGAKU supercomputer and found that magnetic fields reach magnetar strength within milliseconds of the collision, SciTechDaily reports. Each star in the model had 1.35 times the Sun's mass, and the calculation reached a spatial resolution of 6.25 meters scitechdaily.com
SciTechDailyscitechdaily.comOpen source ↗. The work was done by Kenta Kiuchi, Alexis Reboul-Salze, Yuichiro Sekiguchi and Masaru Shibata.
Simulation and magnetic field growth
A collision of two neutron stars often ends in a black hole, but some mergers leave a massive neutron star with a very strong magnetic field. Magnetars are a rare class of neutron stars with fields far stronger than those of ordinary ones. They are associated with bursts of radiation, including soft gamma-ray repeaters, and may also produce some fast radio bursts.
The simulated stars started with fields of approximately 3.16 trillion gauss, consistent with the upper range observed in binary pulsars. The collision produced fields of around 10 quadrillion gauss across parts of the remnant, with localized regions reaching greater strengths. Within approximately five milliseconds, the stellar-scale field rose from 3.16 trillion gauss to around one quadrillion gauss, which is magnetar strength.
Mechanism and limitations
The authors attribute the growth to the Kelvin-Helmholtz instability, which develops when neighboring layers of fluid move at different velocities. According to the authors, it amplifies the field to an expected electromagnetic saturation energy of ∼ 10⁵⁰ erg within 3 ms after the merger and induces stellar-scale amplification by at least a factor of 316 scitechdaily.com
SciTechDailyscitechdaily.comOpen source ↗. The strongest amplification first occurs on very small scales, and the turbulent field then influences progressively larger regions of the merged star.
The authors caution that uncertainties in turbulence and magnetic dissipation at extremely small scales could affect the estimated amplification. In one extrapolation to more realistic conditions, the stellar-scale field could be overestimated by a factor of 10. Even then, it would still reach approximately 100 trillion gauss.
The magnetic structure also stayed concentrated near the equatorial plane of the remnant rather than forming the organized polar field associated with launching relativistic jets. Additional processes, particularly the magnetorotational instability, may be needed for that configuration. The authors say the result suggests that magnetar formation could be inevitable in binary neutron star mergers, at least temporarily, unless the remnant promptly collapses to a black hole.
The simulation covers only the earliest moments after the collision. Longer calculations are needed to determine how these fields evolve, whether they help power energetic outbursts, and how long the neutron star can survive before possibly collapsing into a black hole.
