Neutron stars collide and create a neutron star heavier than previously thought possible

Astronomers witnessed a flash of gamma radiation caused by the collision of two neutron stars, but analysis has thrown some of what we believe we know of astrophysics out the window.

Just when physics begins to get a handle on how things work, the universe inevitably throws a curve ball.

Neutron stars are cosmic heavyweights. They are among the densest objects in the universe, reaching about 1-3 times the mass of our Sun, but little more than 20 kilometres in diameter.

These compact stellar objects are formed when a supergiant star runs out of fuel, causing its core to collapse. This collapse pushes electrons and protons in the supergiant’s core together so tightly that they merge to become neutrons.

Any larger than 25 solar masses, and a star which dies no longer leaves behind a neutron star, but a black hole. The additional mass leads to an object so dense that not even light can escape the black hole’s gravitational pull.

So, theoretically, two neutron stars colliding – combining their masses – should form a black hole. But not so.

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In research published in the Astrophysical Journal, the gamma ray burst from two colliding neutron stars led to the formation of a highly-magnetised neutron star far heavier than the widely-accepted maximum possible mass of a neutron star.

Such a system shouldn’t exist, but scientists observed the juggernaut neutron star surviving for over a day before it collapsed down into a black hole.

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

“They’re such weird exotic objects,” co-author Professor Carole Mundell, also at Bath told the Guardian. “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.”

The gamma ray burst which has caused all this fuss was detected in June 2018 and designated GRB [Gamma-Ray Burst] 180618A. Occurring 10.6 billion light years from Earth, the explosion of the neutron star pair’s collision was observed in three stages: the burst, a kilonova explosion (caused by colliding neutron star binaries), and the afterglow.

The astronomers noticed that the afterglow stopped emitting light 35 minutes after the initial burst. This was because the explosion was being propelled at close to lightspeed by a continuous energy source – this is consistent with a neutron star, not a black hole.

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Not only was the neutron star massive, it was a specific type of neutron star called a magnetar. The object had a magnetic field 1,000 times more powerful than that of an ordinary neutron star and a quadrillion (one with 15 zeroes after it) more powerful than Earth’s magnetic field.

The magnetar lived for nearly 28 hours.

“For the first time, our observations highlight multiple signals from a surviving neutron star that lived for at least one day after the death of the original neutron star binary,” says Jordana-Mitjans.

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

What caused GRB 180618A to result in such a long-lived “supramassive” magnetar is unclear and will be investigated further. The team suggest that its powerful 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 duration gamma-ray bursts are coming from black holes.

“Such findings are important as they confirm that newborn neutron stars can power some short-duration GRBs and the bright 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 thus gravitational waves emitters, when we’re searching the skies for signals.”