Astronomers have witnessed a gamma-ray flash from the collision of two neutron stars, but the analysis has thrown away some of what we think we know about astrophysics.
Just when physics starts to get a handle on how things work, the universe inevitably throws a curveball.
Neutron stars are cosmic heavyweights. They are among the densest objects in the universe, with a mass of about 1-3 times that of our sun, but a diameter of just over 20 kilometers.
These compact stellar objects form when a giant star runs out of fuel, causing its core to collapse. This collapse pushes the electrons and protons in the core of the supergiant together so tightly that they fuse to become neutrons.
Any solar mass greater than 25, and a star that dies no longer leaves behind a neutron star, but rather a black hole. The extra mass results in an object so dense that not even light can escape the black hole’s gravitational pull.
So, in theory, two neutron stars – adding their masses – should collide with a black hole. But it is not.
In research published in Astrophysical JournalA gamma-ray burst from two colliding neutron stars creates a hypermagnetic neutron star much heavier than the widely accepted maximum mass of a neutron star.
Such a system should not have existed, but scientists have observed the juggernaut neutron star surviving for more than a day before collapsing into a black hole.
“Such a massive neutron star with such a long life expectancy is not usually thought possible,” says first author Dr Nuria Jordana Mitjans, an astronomer at the University of Bath. guardian. “It’s a mystery why it was so long-lived.”
“It’s weird weird stuff,” said Study Co-Professor Carol Mundell, also in Bath. guardian. “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.”
The gamma ray burst causing all the fuss was discovered in June 2018 and the GRB identified [Gamma-Ray Burst] 180618 a. Occurring 10.6 billion light-years from Earth, the neutron-couple collision explosion was observed in three phases: explosion, kilonova explosion (caused by colliding neutron-star binaries), and afterflare.
Astronomers noted that the afterglow stopped emitting light 35 minutes after the initial burst. This was because the explosion was propelled near the speed of light by a continuous energy source – this corresponds to a neutron star, not a black hole.
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Not only was the neutron star massive, but a specific type of neutron star called a magnetar. The object has a magnetic field 1,000 times stronger than the magnetic field of a normal neutron star and a quadrillion (one with 15 zeros after it) stronger than Earth’s magnetic field.
The magnetar lived for approximately 28 hours.
“For the first time, our observations highlight multiple signals from a surviving neutron star that lived for at least 1 day after the death of the original neutron star binary,” says Jordana-Mitjans.
“This is the first direct glimpse we might get of a very massive neutron star in nature,” Mundell adds. “My hunch is we’ll find more of them.”
What caused GRB 180618A to produce such a long-lived “supermassive” magnetar is not clear and will be investigated further. The team suggests that its strong magnetic field may have caused an external force that, at least for a time, prevented the material from collapsing further.
This indicates that we can no longer assume that short-period gamma-ray bursts come from black holes.
“Results like these are important because they confirm that infant neutron stars can power some of the short-period GRBs and the bright emissions across the electromagnetic spectrum that accompany them have been detected,” Mundell continues in the article. guardian. “This discovery may provide a new way to locate neutron star mergers, and thus emitters of gravitational waves, when we search the sky for signals.”