A surprisingly bright cosmic explosion could have marked the birth of a magnetar. If so, it would be the first time astronomers have seen this type of fast-spinning, extremely magnetized star body emerge.
This dazzling flash of light was created when two neutron stars collided and merged into a massive object, astronomers report in an upcoming issue of Astrophysical Journal. Although the extra bright light could mean a magnetar was generated, other explanations are possible, the researchers say.
Astrophysicist Wen-fai Fong of Northwestern University in Evanston, Illinois, and colleagues first discovered the location of the neutron star crash as a burst of gamma-ray light discovered on May 22 with NASA’s orbiting Neil Gehrels Swift Observatory, X-ray, visible and infrared wavelengths of light indicated that the gamma rays were accompanied by a characteristic glow known as the kilonova.
Kilonovas are believed to form after two neutron stars, the ultra-dense cores of dead stars, collide and merge. The fusion sprays the collision site with neutron-rich material that “can’t be seen anywhere else in the universe,” says Fong. This material quickly creates unstable heavy elements, and these elements soon decay, heating the neutron cloud and making it glow in optical and infrared light (SN: 10/23/19).
Astronomers believe that every time a pair of neutron stars merge, kilonovas form. But fusions also produce other, brighter light that can flood the Kilonova signal. As a result, astronomers only saw one final kilonova in August 2017, although there are other potential candidates (SN: 10/16/17).
However, the glow that Fong’s team saw put the Kilonova to shame in 2017. “It’s possibly the brightest Kilonova we’ve ever seen,” she says. “It basically breaks our understanding of the luminosity and brightness that kilonovae are supposed to have.”
The greatest difference in brightness was in infrared light, measured with the Hubble Space Telescope about 3 and 16 days after the outbreak of gamma rays. This light was ten times as bright as infrared light seen in previous neutron star fusions.
“That was the real moment that opened our eyes and then we tried to find an explanation,” says Fong. “We had to come up with an additional source [of energy] that got this Kilonova going. “
Her favorite explanation is that the crash created a magnetar, which is a type of neutron star. When neutron stars merge, the mega neutron star they produce is usually too heavy to survive. Almost immediately, the star succumbs to strong gravitational forces and creates a black hole.
However, if the supermassive neutron star is spinning rapidly and has a strong magnetic charge (in other words, it is a magnetar), it could save itself from collapse. Both supporting its own rotation and releasing energy and thus some mass into the surrounding neutron-rich cloud could prevent the star from turning into a black hole, the researchers suggest. That extra energy would, in turn, cause the cloud to give off more light – the extra infrared light that Hubble discovered.
But there are other possible explanations for the extra bright light, says Fong. If the colliding neutron stars had created a black hole, that black hole could have triggered a beam of charged plasma moving at near the speed of light (SN: 02/22/19). The details of how the jet interacts with the neutron-rich material around the collision site could also explain the additional Kilonova glow, she says.
If a magnetar were created, “it could say something about the stability of neutron stars and how massive they can get,” says Fong. “We don’t know the maximum mass of neutron stars, but we do know that in most cases they would fall into a black hole [after a merger]. If a neutron star has survived, it tells us under what conditions a neutron star can exist. “
It would be exciting to find a baby magnetar, says astrophysicist Om Sharan Salafia of the Italian National Institute for Astrophysics in Merate, who was not involved in the new research. “A newborn, highly magnetized, highly rotating neutron star that forms from the fusion of two neutron stars has never been observed before,” he says.
However, he agrees that it is too early to rule out other explanations. Recent computer simulations also suggest that it could be difficult to see a newborn magnetar even if it had formed, he says. “I wouldn’t say that’s finished.”
Fong and her colleagues calculated how the light from the object would behave over the next four months to six years to prove whether a magnetar was born or not.
Fong himself plans to pursue the mysterious object for a long time with existing and future observatories. “I’ll probably follow this until I’m old and gray,” she says. “I will train my students and their students to do this.”