Key takeaways

  • Astronomers found a massive neutron star formed from two smaller ones, challenging existing theories.
  • Unlike usual, this neutron star stayed intact for over a day instead of collapsing into a black hole.
  • The neutron star, not a black hole, caused a powerful gamma-ray burst, suggesting a different process at work.
  • Neutron stars are incredibly dense and strong; this one might have avoided collapse due to fast spinning and strong magnetic fields.
  • This discovery opens up possibilities for finding more such stars and learning about their internal structure with upcoming sky surveys.

Astronomers have discovered a “overweight” neutron star, which they believe challenges existing astronomical ideas.

The supermassive star was formed by the merger of two smaller neutron stars. Normally, such collisions produce neutron stars that are so massive that they collapse into black holes very instantly due to their own gravity. However, the most recent measurements revealed that the monster star remained visible for more than a day before fading away.

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

The findings also raise questions regarding the origins of very powerful flashes known as brief gamma-ray bursts (GRBs), which occur during neutron star mergers. These outbursts, the most explosive events in the cosmos since the big bang, were commonly considered to originate at the poles of the freshly formed black hole. However, in this scenario, the reported gamma-ray burst must have come from the neutron star itself, indicating that a completely other process was at work.

Neutron stars are the tiniest and densest stars known to exist, occupying a sweet spot between ordinary and black holes. They are around 12 miles broad and so dense that a teaspoon of material has a mass of one billion tonnes. They have a smooth crust of pure neutrons, which is 10 billion times stronger than steel.

“They’re such weird exotic objects,” said Prof Carole Mundell, an astronomer at the University of Bath and research co-author. “We can’t gather 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.”

Mundell believes that something stopped the neutron star from “noticing how massive it is” in this scenario. One hypothesis is that the star was spinning so fast and had such powerful magnetic fields that its collapse was postponed, similar to how water stays inside a tilted bucket if swung around quickly enough.

“This is the first direct glimpse that we may have of a hypermassive spinning neutron star in nature,” Mundell told the crowd. “My hunch is we’ll be finding more of them.”

The surprising observations were made using NASA’s orbiting Neil Gehrels Swift Observatory, which caught the initial gamma-ray burst from a galaxy approximately 10.6 billion light years away. The Liverpool Telescope, a robotic observatory located in the Canary Islands, then swiveled mechanically to see the aftermath of the merger. These investigations revealed the unmistakable signs of a highly magnetized, fast spinning neutron star.

This shows that the gamma-ray burst was caused by the neutron star itself, rather than by gravitational collapse. Until now, the exact sequence of events has been difficult to determine.

“We were excited to catch the very early optical light from this short gamma-ray burst – something that is still largely impossible to do without using a robotic telescope,” Mundell told the press. “Our discovery opens new hope for upcoming sky surveys with telescopes such as the Rubin Observatory LSST, with which we may 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 Astronomical Observatory in Milan who was not involved in the study, stated, “The team discovered evidence of the existence of a meta-stable hypermassive neutron star, which is a really important finding.”

He stated that the findings could reveal fresh insights into the interior structure of neutron stars, which are thought to have a core of unusual matter, albeit the specific shape is unclear.

The findings are published in the Astrophysical Journal.

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