July 29, 2013
New Simulations Shed Light On Power Magnetic Fields And Gamma-ray Bursts
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
Gamma-ray bursts (GRBs) are thought to be the most powerful, energetic events in the Universe. They can release as much energy in one second as an entire galaxy expels in an entire year. But while astronomers have some solid theories on what drives these events, many of the details are still widely debated.
One of the complicating matters comes from the fact GRBs appear to fall into two separate classes: long and short burst events. The shorter variety is more intense, yet their brightness falls off more quickly. While longer bursts follow the opposite pattern and have different spectral characteristics.
In the case of short bursts - the ones that will be focused upon here - the prevailing theory is they are initiated by the merging of two neutron stars. As the two dense objects orbit each other at rapidly decaying orbits, eventually their intense gravitational fields begin to tear at one another, their powerful magnetic fields releasing energy as they approach.
Eventually, the two stellar remnants merge into a single hyper-massive neutron star, if only momentarily. Because of their immense combined mass, the newly formed neutron star cannot balance the incredible gravitational force and begins to collapse into a black hole. It is in this collapse process we witness a GRB.
There are issues, however. Specifically, the energetics of the collapse don't quite work because previous simulations suggest the magnetic field of the massive neutron star is not sufficient. But new research has found that instabilities in the core of the neutron star can lead to the creation of very powerful magnetic fields, strong enough to account for the massive outflow of energy seen in these events.
Examining the plasma layers within the massive neutron star, a research team from the Max Planck Institute for Gravitational Physics found when rotating at different speeds, the plasma would rub together. This action could produce turbulent motion, ultimately resulting in the production of highly amplified magnetic fields - a phenomenon known as magnetorotational instability.
In and of itself, the magnetic amplification mechanism is not novel - it was been predicted in several different types of astrophysical systems - but it is the first time researchers have shown it would be possible in a merging binary system, a first for such systems where there is no analytical framework from which the instability would be predicted to arise. And of course, it is a significant step forward in understanding the nature of short duration gamma-ray bursts.
Results of their research are published in the journal Physical Review D.