‘Overweight’ Neutron Star Challenges Black Hole Theory, Astronomers Say | black holes

An overweight neutron star has been spotted by astronomers, who say the mysterious object is confounding astronomical theories.

The hypermassive star was produced by the merger of two smaller neutron stars. These collisions typically result in neutron stars so massive that they collapse into a black hole almost instantaneously under their own gravity. But the latest observations revealed that the monster star hovered in view for more than a day before it disappeared from view.

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

The observations also raise questions about the source of 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 widely assumed to have been 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 an entirely different process.

Neutron stars are the smallest and densest stars in existence, occupying a sweet spot between conventional 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 crust 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, it appears that something 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, sort of like how water stays inside a tilted bucket if it’s spun fast enough.

“This is the first direct view we can get of a rotating hypermassive neutron star in nature,” Mundell said. “My hunch is that we’re going to find more.”

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

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

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

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

The findings are published in the Astrophysics Magazine.