Awed by a magnetar


Astrophysicists have detected one of the brightest and most enigmatic objects in the Universe. Belinda Smith reports.


An illustration of a gamma-ray burst (white line) and supernova (red cloud) powered by a magnetar. – European Southern Observatory

Some 6.3 billion years ago, in a galaxy far from Earth, a monster star died with a colossal bang. In a few hours, it blasted out more than 500 times the energy the Sun will churn through in its lifetime. What could have powered such a gargantuan explosion? A rare and exotic celestial object known as a magnetar, says an international team of astrophysicists in a study published in Nature in July.

“Since a long-duration gamma-ray burst is produced once every 10,000 to 100,000 supernovae, the star that exploded must be somehow special,” says Jochen Greiner, an astrophysicist at Germany’s Max Planck Institute for Extraterrestrial Physics and lead author of the paper.

Gamma-ray bursts are the high-energy light produced by powerful cosmic explosions. In most cases, the longer the gamma-ray burst, the more energetic its source. Astrophysicists consider any burst lasting more than two seconds a “long gamma-ray burst”. But the gamma ray event detected by NASA’s Swift telescope on 9 December 2011 lasted more than seven hours.

Magnetars are mysterious and exotic objects. Astronomers have

detected fewer than 30.

Only four of these ultra-long gamma-ray bursts have ever been recorded. Astrophysicists think they’re produced by the cataclysmic deaths of supergiant stars up to 70 times the mass of the Sun. Having burned up all available fuel, the star collapses, then explodes spectacularly in a giant supernova. All that's left is a freshly formed black hole, sitting in a shell of hydrogen, helium and “metals” (the name astronomers give any element heavier than helium).

But when Greiner and his colleagues studied the 2011 ultra-long gamma-ray bursts – named GRB 111209A – this explanation didn’t stack up.

Typically, when a supergiant star collapses, massive jets of materials rich in metals – particularly nickel-56 – erupt from around the newly formed black hole. And nickel-56 decays rapidly, releasing just the kind of ultra-long burst of gamma-rays spotted by Swift.

In GRB 111209A’s brilliant afterglow, astronomers at the European Southern Observatory’s La Silla and Paranal Observatories in Chile detected what appeared to be “absurdly high” levels of nickel-56, Greiner says – but puzzlingly low levels of all other metals that usually jet out alongside the nickel.

What could produce these contradictory observations? The team found the combination of exceptionally high brightness and low metal content could result when a magnetar, rather than a black hole, forms in a supernova’s centre.

Magnetars are mysterious and exotic objects. Astronomers have detected fewer than 30. They’re the most strongly magnetised objects in the Universe, with a magnetic field around a thousand trillion times stronger than Earth’s. They are extremely dense – the mass of our Sun squeezed into a sphere only 30 kilometres in diameter – and spin hundreds of times each second.

Why magnetars spin so fast is a mystery. But what we do know is that they lose their spin quickly, Greiner says: “We can watch the spin get slower and slower over the years.” And as a newly formed magnetar starts to slow, astrophysicists think it shrugs off excess energy by emitting a huge pulse of radiation in the form of gamma-rays - explaining the Swift telescope observation. And as it continues to slow, more energy is released at longer wavelengths. Some of that energy happened to match the wavelength typically emitted by nickel-56, explaining the “absurdly high” nickel signal the La Silla Observatory appeared to have detected.

Why do some supergiant stars collapse to form magnetars? “This is the million dollar question,” says Simon Clark, an astrophysicist at the Open University in the UK.

One theory, he says, is that the supergiant star that spawned it already had an unusually strong magnetic field, which became highly concentrated when the star collapsed at the end of its life. But the theory is far from perfect, as recent star surveys don’t show enough stars with sufficiently strong magnetic fields to explain the number of magnetars observed.

But while magnetars remain an enigma, Greiner’s finding “brings us much closer to a new and clearer picture of the workings of gamma-ray bursts”, Clarke says. And as telescopes pick up more ultra-long gamma-ray bursts, they might offer more of a glimpse into the magnetars’ secrets.

  1. http://www.nature.com/nature/journal/v523/n7559/full/nature14579.html
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