Astronomers have noted the presence of “overweight” neutron stars, which they say the mysterious object is puzzling astronomical theory.
Supermassive stars are produced by the merger of two smaller neutron stars. Such collisions usually produce neutron stars so massive that they collapse into black holes almost instantly under their own gravity. But recent observations have revealed that the monstrous star hovered above the horizon for more than a day before disappearing from view.
“Such a massive neutron star with a long life expectancy is usually not considered likely,” said Dr. Nuria Jordana Mitjans, astronomer at the University of Bath. “It’s a mystery why he lives so long.”
The observations also raise questions about the source of the extraordinary energetic 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 – erupted from the pole of a newly formed black hole. But in this case, the observed gamma-ray burst must have come from the same neutron star, indicating an entirely different process.
Neutron stars are the smallest and densest stars that have ever existed, occupying an extraordinary place between conventional stars and black holes. It is about 12 miles wide and so dense that a teaspoon of material has the mass of a billion tons. They have smooth shells of pure neutrons, 10 billion times as much stronger than steel.
“These are strange, strange things,” 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 to 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 “registering how massive it is.” One possibility is that the star is spinning so fast and with such a strong magnetic field that it delays its collapse — something like water that sits in a tilted bucket if it’s shaken fast enough.
“This is the first direct view we can get of a supermassive neutron star in nature,” Mondel said. “My gut feeling is that we will find more.”
The unexpected observations were made using Neil Gehrell’s Swift Observatory, orbiting NASA, which detected a first burst of gamma rays from a galaxy about 10.6 billion light-years away. An automatic observatory, the Liverpool Telescope, located in the Canary Islands, then rotates automatically to view the blending effect. These observations reveal the signatures of a rapidly rotating hypermagnetic neutron star.
This suggests that the neutron star itself is releasing a gamma-ray burst, rather than occurring after its gravitational collapse. Until recently it was difficult to know the exact sequence of events.
“We are very excited about capturing the first optical light from these short bursts of gamma rays, something that would still be entirely impossible without robotic telescopes,” Mondel said. “Our discovery opens new hope for future surveys of the sky using telescopes like the LSST Rubin Observatory, where we can find signals from hundreds of thousands of these 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 study.
He said the work could provide new insights into the internal structure of a neutron star, which may contain a core of exotic matter, although its exact shape is unknown.
The results have been published in Journal of Astrophysics.
