Dynamical stability of Magnetic Supramassive Neutron Stars

Neutron stars are the densest objects in the universe, the collapsed cores of massive stars that have exploded. A single teaspoon of their material would weigh as much as a mountain. These cosmic remnants spin incredibly fast, sometimes hundreds of times per second, and many are threaded with magnetic fields so powerful they could erase a credit card from halfway across the solar system. When two neutron stars collide, they can merge to form a single, even more massive object. If this new star is heavy enough to collapse under its own gravity but is held up by its rapid spin, it is called a supramassive neutron star. These objects are fleeting and unstable; they exist in a precarious balance between the crushing force of gravity and the outward push of rotation. Understanding how these stars behave, especially when their magnetic fields are at their most extreme, helps astronomers decode the violent signals they send across the cosmos, from the ripples in space-time to the flashes of high-energy light.

A team of researchers has now used powerful supercomputers to simulate what happens when these supramassive stars are subjected to magnetic fields as strong as one hundred quadrillion Gauss. To do this, they built digital models of these stars using the laws of physics that govern space, time, and matter, and then watched how they evolved over time. They started with stars that were spinning near the limit of what is possible before they would fly apart, and they injected them with magnetic fields ranging from the strength of a typical magnetar to the extreme fields expected right after a stellar collision. The goal was to see if these magnetic forces would help the stars survive longer or if they would push them over the edge into a catastrophic collapse.

The simulations revealed that the magnetic fields inside these stars are not static; they are chaotic and constantly changing. Initially, the magnetic fields were set up in a simple, orderly pattern, but this arrangement is unstable. Within a fraction of a second, the fields began to tangle and twist, reorganizing themselves into a complex mix of loops and spirals. This process happens so quickly that it is faster than the time it takes for the star to complete a single rotation. As the magnetic fields twisted, they acted like a cosmic brake, grabbing the fast-spinning core of the star and slowing it down while simultaneously speeding up the outer layers. This created a state where the center of the star spun much slower than the surface, a condition known as differential rotation.

For stars that were not quite heavy enough to collapse immediately, this magnetic reshuffling led to a new, stable state. The star did not fall apart, nor did it collapse into a black hole. Instead, it settled into a long-lived configuration where the core and the surface spun at different rates, held in place by the magnetic stresses. However, for the stars that were already on the brink of collapse, the magnetic forces acted as the final trigger. By dragging angular momentum away from the core, the magnetic fields removed the support that was keeping the star's center from crushing down. This caused the core to shrink and the star to collapse into a black hole much faster than it would have without the magnetic influence.

When these stars did collapse, the resulting black holes were surprisingly different from what one might expect. Although the black holes spun faster than the original stars—some rotating nearly fifty percent faster—they actually possessed less total spin energy. This is because the violent collapse and the magnetic fields ejected a significant amount of matter from the star before it disappeared. This ejected material, carrying away a portion of the star's spin, prevented that energy from being absorbed by the new black hole. The simulations showed that this ejected matter formed winds that flowed out primarily around the star's equator, carrying away mass at rates of up to a few solar masses per second, though the total amount lost over the simulated time was a tiny fraction of the star's total weight, on the order of 10⁻³ solar masses.

The study also highlighted a critical difference between stars with weaker magnetic fields and those with the extreme fields found in the most violent cosmic events. In stars with weaker fields, the magnetic forces were too slow to disrupt the star's rotation significantly, allowing the magnetic field to grow stronger over time without causing immediate collapse. In contrast, the stars with the strongest magnetic fields experienced a rapid and turbulent reorganization that either stabilized them in a new state or hastened their demise. The researchers found that the fate of these stars depends on a delicate balance between how fast they spin, how dense they are, and how strong their magnetic fields are. If the magnetic field is strong enough and the star is close to its maximum mass limit, the magnetic forces can tip the scales, turning a stable star into a black hole in the blink of an eye.

These findings offer a new perspective on the life cycle of neutron stars and the aftermath of their collisions. They suggest that the magnetic fields in these objects are not just passive features but active agents that can reshape the star's structure, alter its spin, and determine whether it survives or dies. The simulations also provide a potential explanation for certain types of bright, long-lasting flashes of energy observed in the universe, which might be powered by these newly formed, differentially rotating stars before they finally collapse. While the computer models used in this study are limited to the first few moments of the star's life, they provide a crucial glimpse into the complex interplay of forces that govern the most extreme environments in the universe. The work underscores that even in the vacuum of space, magnetic fields can be powerful enough to rewrite the destiny of a star.