Astronomers witnessed a flash of gamma radiation caused by the collision of two neutron stars, but the analysis threw out some of what we think we know about astrophysics.
Just when physics begins to get a grip on how things work, the universe inevitably throws a curveball.
Neutron stars are cosmic heavyweights. They are among the densest objects in the universe, reaching about 1 to 3 times the mass of our Sun, but just over 20 kilometers in diameter.
These compact stellar objects form when a supergiant star runs out of fuel, causing its core to collapse. This collapse pushes electrons and protons from the supergiant’s nucleus so tightly that they fuse to become neutrons.
Anything larger than 25 solar masses, and a star that dies no longer leaves behind a neutron star, but a black hole. The extra mass leads to an object so dense that not even light can escape the black hole’s gravitational pull.
So, theoretically, two colliding neutron stars – combining their masses – should form a black hole. But not so.
In research published in the Astrophysical Journalthe gamma-ray burst of two colliding neutron stars led to the formation of a highly magnetized neutron star far heavier than the widely accepted maximum possible mass of a neutron star.
Such a system shouldn’t exist, but scientists have observed the behemoth neutron star survive for more than a day before collapsing into a black hole.
“Such a massive neutron star with a long lifespan is not normally thought to be possible,” said first author Dr Nuria Jordana-Mitjans, an astronomer at the University of Bath. Guardian. “It’s a mystery why this one has lived so long.”
“These are such strange alien objects,” said co-author Professor Carole Mundell, also in Bath. Guardian. “We can’t collect this material and bring it back to our lab, so the only way to study it is when they do something in the sky that we can observe.”
The gamma-ray burst that caused all the fuss was detected in June 2018 and designated GRB [Gamma-Ray Burst] 180618A. Occurring 10.6 billion light-years from Earth, the colliding explosion of the pair of neutron stars was observed in three stages: the burst, a kilonova explosion (caused by the collision neutron star binaries) and afterglow.
Astronomers noticed that the afterglow stopped emitting light 35 minutes after the initial burst. Indeed, the explosion was propelled at a speed close to light by a source of continuous energy – which corresponds to a neutron star and not to a black hole.
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Not only was the neutron star massive, but it was a specific type of neutron star called a magnetar. The object had a magnetic field 1,000 times stronger than that of an ordinary neutron star and a quadrillion (one with 15 zeros after) stronger than Earth’s magnetic field.
The magnetar lived nearly 28 hours.
“For the first time, our observations highlight multiple signals from a surviving neutron star that lived at least a day after the death of the original neutron binary star,” says Jordana-Mitjans.
“This is the first direct glimpse we can have of a rotating hypermassive neutron star in nature,” Mundell adds. “My hunch is that we will find others.”
Why GRB 180618A resulted in such a long-lived “supermassive” magnetar is unclear and will be investigated further. The team suggests that its strong magnetic field may have caused an outward force preventing, at least for a time, the material from collapsing further.
This indicates that we can no longer assume that short-lived gamma-ray bursts come from black holes.
“Such findings are important because they confirm that newborn neutron stars can power certain short-lived GRBs and light emissions across the electromagnetic spectrum that have been detected accompanying them,” Mundell continues in the Guardian. “This discovery may offer a new way to locate neutron star mergers, and therefore gravitational wave emitters, when we search for signals in the sky.”
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