Colliding neutron stars create a neutron star that we thought was too heavy to exist
Once again, flashes of light from colliding neutron stars have upended our understanding of how the universe works.
Analysis of the short gamma-ray burst at the merger of the two stars revealed that instead of forming a black hole, as expected, the immediate product of the merger was a highly magnetized neutron star much heavier than the estimated maximum mass of the neutron star.
It appears that this magnetar lasted for more than a day before it collapsed into a black hole.
“Such a massive neutron star with such a long life expectancy is not usually thought possible,” said astronomer Nuria Jordana Mitjans of the University of Bath in the UK. Watchman. “It’s a mystery why it was so long-lived.”
Neutron stars are on the spectrum of how a star might end up at the end of its life. For millions or billions (or maybe even trillions) of years, the star will oscillate, a motor fusing the atoms in its hot, compressed core.
Eventually, the star will run out of atoms that can fuse, and at this point, the whole thing explodes. The star ejects its outer mass, and no longer supported by the outward pressure provided by fusion, the core collapses under the inward pressure of gravity.
How these collapsed nuclei are classified depends on the mass of the body. The cores of stars that started out with up to about 8 times the mass of the sun collapse into white dwarfs, which have a maximum mass of 1.4 solar masses, which are compressed into a sphere the size of Earth.
The cores of stars between 8 and 30 solar masses transform into neutron stars, between 1.1 and 2.3 solar masses, in a sphere just 20 kilometers (12 miles) across. According to the theory, larger stars, which exceed the maximum mass of a neutron star, collapse into black holes.
But there is a very noticeable dearth of black holes less than 5 solar masses, so what happens in this mass system is largely a mystery.
This is why neutron star mergers are so exciting to astronomers. Occurs when two neutron stars in a binary system have reached the point of orbital decay where they coalesce into a single neutron star body.
Most binary neutron stars have a combined mass that exceeds the theoretical maximum mass for neutron stars. The products of these mergers are therefore likely to remain constant within the mass gap of the black hole and the neutron star.
When they collide, binary neutron stars release a burst of high-energy radiation known as a short-period gamma-ray burst. Scientists had thought that these could only be emitted during the formation of a black hole.
But exactly how merging neutron stars turn into a black hole has been a mystery. Does the black hole form instantly, or do the two neutron stars produce a very heavy neutron star that then collapses into a black hole very quickly, a few hundred thousandths of a second after merging?
GRB 180618A was a short-lived gamma-ray burst detected in June 2018, light that has traveled 10.6 billion years to reach us. Jordana-Mitjans and her colleagues wanted to take a closer look at the light emitted by this object: the explosion itself, the kilonova explosion, and the much longer-lived afterglow.
But when they looked at the electromagnetic radiation generated by the event over time, something was off.
The optical emission of the afterglow disappeared 35 minutes after the gamma-ray burst. The team found this was because it was expanding at near the speed of light, accelerated by a continuous power source.
This was consistent not with a black hole, but with a neutron star. And not just any neutron star. It appears to be what we call a magnetar: a field with a magnetic field 1,000 times stronger than that of an ordinary neutron star, and a quadrillion times stronger than that of Earth. It was suspended for more than 100,000 seconds (nearly 28 hours).
Jordana-Mitjans says, “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 binary of the neutron star.”
What could have helped the magnets to live so long is not clear. It’s possible that the magnetic field gave it a little help, providing an outward drag that kept it from collapsing completely, at least for a little while.
Whatever the mechanism—and this will certainly require further investigation—the team’s work shows that massive neutron stars are capable of giving off short-range gamma-ray bursts, and that we can no longer assume the existence of a black hole.
“These findings are important because they confirm that infant neutron stars can trigger some short-period GRBs and bright emissions across the electromagnetic spectrum that accompany them,” says Jordana-Mitjans.
“This discovery may offer a new way to locate neutron star mergers, and thus emitters of gravitational waves, when we search the sky for signals.”
Research published in Astrophysical Journal.
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