The Most-Magnetic Objects in the Universe Attract New Controversy
Quanta Magazine said:Magnetars have long been an astrophysical mystery. These compact hunks of nuclear matter — the ultradense remnants of supermassive stars — generate the most extreme magnetic fields in the universe. Researchers have debated exactly how these magnetic fields come about. But a new study of the explosions that gave rise to magnetars suggests that a long-favored theory might not hold up.
The idea of a magnetar dates back to the early 1990s, when the possibility was explored by astronomers who studied the death of stars. Large stars — those larger than around eight times the mass of the sun — will end their lives in a supernova. These enormous explosions cast out the star’s outer layers in the form of a supernova remnant, leaving behind the dead core of a neutron star.
But not all neutron stars are alike. Some are nondescript. Others, called pulsars, have strong magnetic fields and sport intense beams of light that sweep across space. And some have magnetic fields many times stronger than the norm — quadrillions of times the strength of the sun’s magnetic field. These highly magnetized pulsars, which emit unpredictable bursts of X-rays and gamma rays, are magnetars.
Astronomers originally thought that magnetars acquire their magnetic fields during the course of a supernova. If, when a neutron star first forms, it begins to rotate very fast — as fast as 1,000 revolutions per second — it may trigger turbulent internal motions that amplify its magnetic field in a so-called dynamo effect.
Other mechanisms were proposed over the years as well, but the dynamo model remained the most popular. Then in 2006, Jacco Vink, an astrophysicist now at the University of Amsterdam, compared explosions that gave rise to magnetars with those that give rise to ordinary neutron stars. The dynamo model predicts a more energetic explosion than a typical supernova, but Vink found that the explosions were on a similar scale. The findings came as a surprise to some scientists.
“When I wrote in 2006 this paper, most people only thought about the dynamo model,” Vink said, “because this is how the magnetar idea came about.”
But if the dynamo isn’t the cause, how might a magnetar form? The other leading idea starts with the fact that not all ordinary stars have the same magnetic field strength. Perhaps magnetars are simply the final product of stars with particularly strong fields. Astrophysicists call this the “fossil field” model — a magnetar’s magnetic field is a remnant, or fossil, of the original star’s field. “They’re both reasonable ideas,” said Victoria Kaspi, an astrophysicist at McGill University who studies neutron stars, “but we don’t know which is right.”
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