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Astronomers have observed colliding neutron stars that can form a magnet


A surprisingly bright cosmic explosion could mark the birth of a magnet. If so, it would be the first time astronomers have witnessed the formation of this type of extremely magnetized, rapidly rotating stellar corpse.

That dazzling flash of light occurred when two neutron stars collided and fused into a massive object, astronomers report in an upcoming issue of the Astrophysical Journal. Although especially bright light could mean a magnet has been produced, other explanations are possible, the researchers say.

Astrophysicist Wen-fai Fong of Northwestern University in Evanston, Ill., And colleagues first discovered the site of the neutron star's crash as a gamma-ray burst of light detected with NASA's Neil Gehrels Swift Observatory in orbit. May 22. Follow-up observations in X-ray wavelengths, visible and infrared showed that gamma rays were accompanied by a characteristic brightness called kilonova.

Kilonovae are thought to form after two neutron stars, the ultra-dense nuclei of dead stars, collide and merge. Fusion pulverizes neutron-rich material "that is not seen anywhere else in the universe" around the crash site, Fong says. That material quickly produces unstable heavy elements and those elements decay quickly, heating the neutron cloud and making it glow with optical and infrared light (SN: 23/10/19).

A new study found that two neutron stars collided and fused, producing an especially bright flash of light and possibly creating a kind of extremely magnetized, rapidly rotating stellar corpse called a magnet (shown in this animation).

Astronomers think that kilonovae form each time a pair of neutron stars fuse. But fusions also produce another brighter light that can flood the kilonova signal. As a result, astronomers have seen only one definitive kilonova before, in August 2017, although there are other potential candidates (SN: 16/10/17).

The glow that Fong’s team saw, however, embarrassed the 2017 kilonova. “It’s potentially the brightest kilonova we’ve ever seen,” he says. "It basically breaks our understanding of the luminosities and glitters that kilonovas are supposed to have."

The largest difference in brightness was in infrared light, measured by the Hubble Space Telescope about 3 to 16 days after the gamma-ray burst. That light was 10 times brighter than the infrared light seen in previous neutron star fusions.

“That was the real moment that opened our eyes, and that’s when we looked for an explanation,” Fong says. "We had to come up with an extra (energy) source that would drive that kilonova."

His favorite explanation is that the accident produced a magnet, which is a type of neutron star. Typically, when neutron stars melt, the mega-neutron star they produce is too heavy to survive. Almost immediately, the star succumbs to intense gravitational forces and produces a black hole.

But if the supermassive neutron star rotates rapidly and is highly magnetically charged (in other words, it is a magnet), it could be saved from collapsing. The researchers suggest that supporting its own rotation and pouring of energy, and therefore some mass, into the cloud rich in surrounding neutrons could prevent the star from turning into a black hole. That extra energy in turn would make the cloud shed more light – the extra infrared brightness that Hubble detected.

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But there are other possible explanations for the extra bright light, Fong says. If the colliding neutron stars produced a black hole, that black hole could launch a charged plasma jet that moved almost at the speed of light (SN: 22/02/19). The details of how the jet interacts with the neutron-rich material surrounding the collision site could also explain the extra brightness of kilonova, she says.

If a magnet were produced, “that could tell us something about the stability of neutron stars and how massive they can get,” Fong says. "We don't know the maximum mass of neutron stars, but we know that in most cases they would collapse into a black hole (after a fusion). If a neutron star survived, tell us about the conditions under which a neutron star can exist."

Finding a magnetic baby would be exciting, says astrophysicist Om Sharan Salafia of Italy’s National Institute of Astrophysics in Merate, who did not participate in the new research. “A highly magnetized and highly rotating newborn neutron star has never been observed to form from the fusion of two neutron stars,” he says.

But he agrees that it is too early to rule out other explanations. What’s more, the latest computer simulations suggest it may be difficult to see a newborn magnet even if it has formed, he says. "I wouldn't say this is resolved."

Observing how the light of the object behaves in the next four months to six years, Fong and his colleagues calculated, will demonstrate whether or not a magnet was born.

Fong herself plans to follow the mysterious object for a long time with existing and future observatories. “I’ll probably follow this until I’m old and gray,” he says. "I'm going to train my students to do it and their students."


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