‘Overweight’ neutron star defies black hole theory, astronomers say | black holes

An “overweight” neutron star has been observed by astronomers, who say the mysterious object confuses astronomical theories.

The hypermassive 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 immediately under their own gravity. But the latest observations revealed that the monster star lingered in plain sight for more than a day before disappearing from view.

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

The observations also raise questions about the source of incredibly energetic flashes, known as short gamma-ray bursts (GRBs), that accompany the merging of neutron stars. It was widely believed that these eruptions — the most energetic events in the universe since the Big Bang — were launched 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 a very different process was going on.

Neutron stars are the smallest, densest stars in existence and are well placed between conventional stars and black holes. They are about 12 miles wide and 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.

“They are such strange exotic objects,” said Prof. 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 air 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 — kind of like how water stays in a tilted bucket if it’s spun fast enough.

“This is the first direct glimpse we can have of a hypermassive spinning neutron star in nature,” Mundell said. “My hunch is that we will find more.”

The unexpected observations were made using 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, the Liverpool Telescope, located in the Canary Islands, then turned on automatically to watch the aftermath of the merger. These observations revealed telltale signatures of a highly magnetized, rapidly spinning neutron star.

This suggests that the neutron star itself launched the gamma-ray burst, rather than it occurring after the 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 short gamma-ray burst — something that’s still largely impossible without a robotic telescope,” Mundell said. “Our discovery opens up new hope for future sky surveys with telescopes like the Rubin Observatory LSST, which will help us find signals from hundreds of thousands of such long-lived neutron stars before collapsing to become black holes.”

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

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

The findings are published in the Astrophysical Journal.