Huge Exploding Stars can be Powered by the Strongest Magnets

Artist’s impression of a gamma-ray burst and supernova powered by a magnetar. ESO.

When a massive star comes to the end of its life, it collapses and explodes in a colossal, energetic event known as a supernova. Occasionally, a star will release a final 'death cry' in the form of a gamma-ray burst (GRB): a powerful stream of high-energy gamma-rays that we can detect from satellites and on Earth. Astronomers used to think that this burst of gamma-rays signaled that the core of the star had collapsed to become a black hole. This process, called the collapsar model, has been accepted for nearly 20 years. However, some scientists think they have discovered another possibility.

A team of astronomers from the Max-Planck Institut für extraterrestrische Physik think they've identified a supernova that released a GRB when it turned not into a black hole, but instead into a strange, celestial object called a magnetar. A magnetar is a type of neutron star that, as the name suggests, has an extremely powerful magnetic field. The strength of this field is about a thousand trillion times stronger than the Earth's magnetic field. This makes them the most powerful magnetic objects in the known universe.

The secret of their magnetism is still a bit of a mystery. After a massive star undergoes a supernova explosion, its core collapses due to its gravitational attraction. This process usually forms a neutron star, which has a magnetic field. However, occasionally they form magnetars instead, which have immense gravitational fields. One theory about their conception is that the magnetar has to be spinning around 100 to 1,000 times per second to generate the energy for this level of magnetism. 

The previously unknown relationship between magnetars, GRBs and supernovae was found due to an epic, high-energy event on December 9, 2011. The Swift satellite found an ultra-long duration gamma-ray burst – one of the most energetic events in the universe. In this case, a massive star going supernova. The GRB was the longest and brightest ever observed; usually GRBs last a few seconds, this one lasted for a few hours.

When measuring the afterglow of the burst, it was noticed that the signals corresponded to that of a supernova. This correlation between an ultra-long GRB and a supernova was a world first. However, there was something unusual about the element signals in the afterglow. While a signal from nickel-56, an element formed in a supernova explosion, was expected, the amount they observed was far too large to be explained by a supernova forming a black hole. Therefore, something else had to be at work. 

The only explanation that fit was that this supernova had formed a magnetar. Jochen Greiner, lead author of the paper published in Nature, told IFLScience that he was extremely surprised by these results. "For nearly 20 years we have got used to the collapsar model (the one with the black hole) which since then has been the standard scenario for the long-duration GRBs... Now it takes some photometry at a 2m telescope and one spectrum to overthrow this conviction."

Greiner summed up the importance of this finding: "Many of us have been specialising on one detailed subject, ignoring what other colleagues have been finding. Now we suddenly find how several of these subjects are connected, i.e. gamma-ray bursts, magnetars, superluminal, supernovae." It's a lesson that every branch of the scientific community can benefit from.

[Via ESO]

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