An “overweight” neutron star has been observed by astronomers, who say the mysterious object confuses astronomical theories.
The hypermassive star was produced by the merger of two smaller neutron stars. Normally, such collisions result in neutron stars so massive that they almost instantly collapse into a black hole under their own gravity. But the latest observations revealed that the monster star hovered in view for more than a day before fading out of sight.
“Such a massive neutron star with a long lifespan is not normally considered possible,” said Dr Nuria Jordana-Mitjans, an astronomer at the University of Bath. “It’s a mystery why this one has lived so long.”
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. These explosions – the most energetic events in the universe since the big bang – were widely assumed to be launched from the poles of the newly formed black hole. But in this case, the observed gamma-ray burst must have emanated from the neutron star itself, suggesting that an entirely different process was at play.
Neutron stars are the smallest and densest stars in existence, occupying a middle ground between conventional stars and black holes. They are about 20 km wide and are so dense that a teaspoon of material would have a mass of 1 billion tons. They have a smooth crust of pure neutrons, 10 billion times stronger than steel.
“These are such strange alien objects,” said Professor Carole Mundell, an astronomer at the University of Bath and co-author of the study. “We can’t collect this material and bring it back to our lab, so the only way to study it is when they do something in the sky that we can observe.”
In this case, Mundell said, something seems to have prevented the neutron star “from noticing how massive it is.” One possibility is that the star was spinning so fast and with such immense magnetic fields that its collapse was delayed – something like the way water stays inside a tilted bucket if swung around fast enough.
“This is the first direct glimpse we can have of a spinning hypermassive neutron star in nature,” Mundell said. “My hunch is that we will find others.”
The unexpected observations were made using NASA’s orbiting Neil Gehrels Swift Observatory, which detected the initial gamma-ray burst originating from a galaxy about 10.6 billion light-years away. A robotic observatory, the Liverpool Telescope, located in the Canary Islands, then automatically rotated to visualize the consequences of the merger. These observations revealed telltale signatures of a highly magnetized, rapidly rotating neutron star.
This suggests that the neutron star itself initiated the gamma-ray burst, rather than occurring after its gravitational collapse. So far, the exact sequence of events has been difficult to figure out.
“We were thrilled to capture the first-ever optical light from this short burst of gamma rays – something that is still largely impossible to do without the aid of a robotic telescope,” Mundell said. “Our discovery opens new hope for future studies of the sky with telescopes such as the Rubin LSST Observatory, with which we could find signals from hundreds of thousands of long-lived neutron stars before they die. collapse to become black holes.”
Stefano Covino, an astronomer at the Brera Astronomical Observatory in Milan, who was not involved in the search, said: “The team found evidence for the existence of a meta-stable hypermassive neutron star, which is a really important finding.”
He said the work could provide new insights into the interior structure of neutron stars, which are thought to have a core of exotic matter, although the exact shape this takes is unknown.
The results are published in the Astrophysical Journal.
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