June 10, 2023

‘Supermassive’ neutron star defies black hole theory, astronomers say

A “supermassive” neutron star has been spotted by astronomers, who say the mysterious object is confounding astronomical theories.

The supermassive star was created by the merger of two smaller neutron stars. Normally such collisions 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 monster star hangs in the eye for more than a day before disappearing from sight.

“Such a massive neutron star with a long life expectancy is not normally considered possible,” said Dr Nuria Jordana-Mitjans, an astronomer at the University of Bath. “It’s a mystery why this lived so long.”

The observations also raise questions about the source of the incredibly energetic flares, known as short gamma-ray bursts (GRBs), that accompany neutron star mergers. These outbursts – the most energetic events in the universe since the Big Bang – were widely thought to be fired from the poles of the newly formed black hole. But in this case, the observed gamma-ray burst must have come from the neutron star itself, suggesting that an entirely different process was at play.

Neutron stars are the smallest, densest stars in existence, occupying a sweet spot between conventional stars and black holes. It is 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 pure neutrons, 10 billion times stronger than steel.

“They are such 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 when they do something in the sky that we can observe.”

In this case, Mundell said, something appears 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 huge magnetic fields that it delayed its collapse – kind of like how water stays inside a tilted bucket if it’s spinning fast enough.

“This is the first direct glimpse we might have of a supermassive rotating neutron star in nature,” Mundell said. “My belief is that we will find more of these.”

The unexpected sightings were made using NASA’s orbiting Neil Gehrels Swift Observatory, which detected the initial gamma-ray burst coming from a galaxy about 10.6 billion light-years away. A robotic observatory, the Liverpool Telescope, located in the Canary Islands, then rotated automatically to view the aftermath of the merger. These observations revealed telltale signatures of a highly magnetized, rapidly rotating neutron star.

This suggests that the neutron star itself launched the gamma-ray burst, rather than occurring after its gravitational collapse. Until now, the exact sequence of events has been difficult to determine.

“We were excited to catch the very early visible light from this brief gamma-ray burst – something that is still largely impossible to do without the use of a robotic telescope,” said Mundell. “Our discovery opens new hopes for future surveys of the sky with telescopes like the Rubin LSST Observatory, with which we can find signals from hundreds of thousands of such long-lived neutron stars before they collapse to become black holes.”

Stefano Covino, an astronomer at the Brera Observatory in Milan, who was not involved in the research, said: “The team found evidence for the existence of a metastable supermassive neutron star, which is a really important finding.”

He said the work could provide new insights into the internal structure of neutron stars, which are assumed to have a core of exotic matter, although the exact form it takes is unknown.

The findings are published in the Astrophysical Journal.

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