Largest Magnetic Fields in Universe Triggered by Neutron Stars

First Posted: Jul 26, 2013 09:06 AM EDT

Neutron stars are some of the densest objects in the universe. Now, scientists have discovered a little bit more about them. Numerical simulations have shown the occurrence of an instability in the interior of these neutron stars that can lead to massive magnetic fields, triggering one of the most dramatic explosions observed in the universe.

Ultra-dense, or hypermassive, neutron stars are formed when two neutron stars in a binary system merge, becoming one entity. Yet this ultra-dense body doesn't last for long. Its short life ends when it catastrophically collapses to become a black hole. Possibly powered by a short gamma-ray burst, this collapse could potentially be formed by one of the brightest explosions observed in the universe. Scientists have long suspected that enormous magnetic field strengths, possibly higher than any observed in any known astrophysical system, are a key ingredient in such an emission. In order to observe whether this was possible or not, though, the researchers turned to simulations.

The scientists simulated a hypermassive neutron star with an initially ordered ("poloidal") magnetic field. Its structure was subsequently made more complex by the star's rotation. Since the star is dynamically unstable, it eventually collapsed to a black hold surrounded by a cloud of matter.

So how would this generate such a huge magnetic field? It could be explained by a phenomenon that can be triggered in a differentially rotating plasma in the presence of magnetic fields. Neighboring plasma layers which rotate and "rub" against each other eventually set the plasma into turbulent motion. In this process, called magnetorotational instability, magnetic fields can be strongly amplified. This particular mechanism plays an important role in many astrophysical systems, which means that it's quite possible that it could be present in hypermassive neutron stars.

In fact, the latest simulations have shown the presence of an exponentially rapid amplification mechanism in the stellar interior--the magnetorotational instability. This shows not only the mechanism in the framework of Einstein's theory of general relativity, but also has a profound astrophysical impact. It supports the idea that ultra-strong magnetic fields can be a key ingredient in explaining the huge amount of energy emitted by short gamma-ray bursts. This, in turn, could help spur further research.

The findings are published in the journal Physical Review D.

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