Astronomers have spotted an “overweight” neutron star, saying the mysterious object has confounded astronomical theories.
A hypermassive star was formed by the merger of two small neutron stars. Normally, such collisions make neutron stars so massive that they almost instantly collapse under their own gravity into a black hole. But recent observations have revealed that the monster star can stay in sight for more than a day without disappearing.
Dr Nuria Jordana-Mitjans, an astronomer at the University of Bath, said: “Such a large, long-lived neutron star is not normally thought to be possible.” “Why it has such longevity is a mystery.”
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 bursts – the most energetic events in the universe since the big bang – were thought to be ejected from the poles of the nascent black hole. However, the gamma-ray burst observed in this case must have originated from the neutron star itself, which indicates that a completely different process is taking place.
Neutron stars are the smallest, densest stars in existence and occupy a sweet spot between ordinary stars and black holes. They are about 12 miles wide and so dense that a teaspoon of the material would have a mass of 1 billion tons. They have a smooth shell of 10 billion times pure neutrons stronger than steelđŸ‡§đŸ‡·
“They are very strange exotic 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 we can study it is if they do something in the sky that we can observe.”
In this case, Mundell said, something is preventing the neutron star from “noticing how big it is.” One possibility is that the star was spinning so fast and with such large magnetic fields that its collapse was delayed—kind of like water staying in a tilted bucket when it spins fast enough.
“This is the first direct observation of a hypermassive rotating neutron star in nature,” Mundell said. “My guess is that we’ll find more of them.”
The unexpected images were taken by NASA’s orbiting Neil Gehrels Swift Observatory, which detected the first gamma-ray burst from a galaxy about 10.6 billion light-years away. A robotic observatory in the Canary Islands, the Liverpool Telescope, automatically rotated to see the results of the merger. These observations revealed the signature signatures of a highly magnetized, rapidly rotating neutron star.
This suggests that the neutron star triggered the gamma-ray burst, not after its gravitational collapse. Until now, it was difficult to understand the exact sequence of events.
“We were excited to capture very early optical light from this short gamma-ray burst—something that is still impossible to do without using a robotic telescope,” Mundell said. “Our discovery opens up new hopes for future sky surveys with telescopes like the Rubin Observatory LSST. It could help us find signals from hundreds of thousands of such long-lived neutron stars before they collapse into black holes.”
“The team has found evidence for the existence of a metastable hypermassive 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 study.
He said the work could provide new insights into the internal structure of neutron stars believed to have a core of exotic matter, although its exact shape is unknown.
The results are published in the journal Journal of AstrophysicsđŸ‡§đŸ‡·
