Astronomers have noticed the presence of an “overweight” neutron star, who say the mysterious object is confusing astronomical theories.
The supermassive star was produced by the fusion of two smaller neutron stars. Such collisions usually result in neutron stars so massive that they collapse into a black hole almost instantaneously under their own gravity. But the latest observations have revealed that the monstrous star hovered over the horizon for more than a day before fading out of sight.
“Such a massive neutron star with a long life expectancy is not usually thought to be possible,” said Dr. Nuria Jordana Mitjans, an astronomer at the University of Bath. “It is a mystery why this was so long-lived.”
The observations also raise questions about the source of the incredibly energetic flashes, known as short gamma-ray bursts (GRBs), that accompany neutron star mergers. It is widely assumed that these outbursts – the most energetic events in the universe since the Big Bang – erupt from the poles of a newly formed black hole. But in this case, the observed gamma-ray burst must have emanated from the neutron star itself, indicating an entirely different process.
Neutron stars are the smallest and densest stars in existence, occupying a remarkable place among conventional stars and black holes. It is about 12 miles wide, and is so dense that a teaspoon of the material has a mass of one billion tons. They have a smooth shell of pure neutrons, 10 billion times stronger than steel.
“It’s weird, weird stuff,” said Professor Carol Mondel, an astronomer at the University of Bath and co-author of the study. “We can’t collect these materials and bring them back to our lab, so the only way we can study them is when they do something in the sky that we can observe.”
In this case, Mondel said, it appears that something prevented the neutron star from “noting how massive it is.” One possibility is that the star was spinning so fast and with such massive magnetic fields that its collapse was delayed – something like water staying inside a tilted bucket if it was swaying fast enough.
“This is the first direct glimpse we might get of a supermassive neutron star in nature,” Mondel said. “My hunch is we’ll find more of them.”
The unexpected observations were made using NASA’s orbiting Neil Gehrells Swift Observatory, which detected an initial gamma-ray burst coming from a galaxy about 10.6 billion light-years away. An automated observatory, the Liverpool Telescope, located in the Canary Islands, then automatically rotated to display the effects of the merger. These observations revealed telltale signs of a rapidly rotating, hypermagnetic neutron star.
This indicates that the neutron star itself unleashed a gamma-ray burst, rather than occurring after its gravitational collapse. Until now, it was difficult to know the exact sequence of events.
“We were excited to capture the very early optical light from this short gamma-ray burst — something that is still pretty much impossible to do without a robotic telescope,” Mondel said. “Our discovery opens new hope for upcoming sky surveys using telescopes such as the LSST’s Rubin Observatory, in which we may find signals from hundreds of thousands of these long-lived neutron stars before they collapse into black holes.”
“The team found evidence of a stable, supermassive neutron star, which is a really important finding,” said Stefano Covino, an astronomer at the Brera Astronomical Observatory in Milan, who was not involved in the research.
He said the work could provide new insights into the internal structure of neutron stars, which presumably contain a core of exotic matter, although the exact form this takes is unknown.
The results have been published in Astrophysical Journal.
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