Astronomers have noted the presence of an “overweight” neutron star, which claims that the mysterious object confuses astronomical theories.
Supermassive stars are produced by the merger of two smaller neutron stars. Such collisions usually produce neutron stars so massive that they collapse into the black hole almost instantly under their own gravity. But recent observations revealed that the terrifying star remained above the horizon for more than a day before disappearing from view.
“Such a massive neutron star with such a long life expectancy is generally not considered possible,” said Dr. Nuria Jordana Mitjans, astronomer at the University of Bath. “It’s a mystery why it’s so long-lived.”
The observations also raise questions about the source of the extraordinary energy bursts, known as short gamma-ray bursts (GRBs), that accompany neutron star mergers. It is widely believed that this explosion – the most energetic event in the universe since the Big Bang – exploded from the pole of a newly formed black hole. But in this case, the observed gamma-ray burst must come from the same neutron star, indicating an entirely different process.
Neutron stars are the smallest and densest stars that have ever existed, and they occupy an extraordinary place among conventional stars and black holes. It is about 12 miles wide and so dense that a teaspoon of material has a mass of one billion tons. They have a thin shell of pure neutrons, 10 billion times stronger than steel.
“It’s weird, weird stuff,” said Professor Carol Mondel, an astronomer at the University of Bath and co-author of the study. “We can’t collect these materials and bring them back to our lab, so the only way we can study them is when they do something in the sky that we can observe.”
In this case, Mondel said, there appears to be something preventing the neutron star from “recording how massive it is.” One possibility is that the star is spinning so fast and with such a large magnetic field that its collapse is delayed, something like water that stays in a tilted bucket if it swings fast enough.
“This is the first direct view we could have of a supermassive neutron star in nature,” Mondel said. “My impression is that we will find more.”
The unexpected observations were made using NASA’s orbiting Neil Gehrells Swift Observatory, which detected an initial gamma-ray burst from a galaxy about 10.6 billion light-years away. An automated observatory, the Liverpool Telescope, located in the Canary Islands, is then automatically rotated to visualize the blending effect. These observations reveal the signs of a rapidly spinning hypermagnetic neutron star.
This suggests that the neutron star itself released a gamma-ray burst, instead of occurring after its gravitational collapse. Until recently, it was difficult to know the exact sequence of events.
“We are very excited to capture the first optical light from these short gamma-ray bursts, something that is still highly impossible without a robotic telescope,” Mondel said. “Our results open new hope for future sky surveys using telescopes such as the LSST Rubin Observatory, where we can find signals from these hundreds of thousands of long-lived neutron stars before they collapse into black holes.”
“The team found evidence of a stable supermassive neutron star, which is a very important discovery,” said Stefano Covino, an astronomer at the Brera Astronomical Observatory in Milan who was not involved in the research.
He said the work could provide new insight into the internal structure of neutron stars, which may contain nuclei of exotic matter, although their exact shape is unknown.
The results were published in Journal of Astrophysics.
