A flash of light emitted by colliding neutron stars has once again altered our understanding of how the Universe works.
Analysis of the brief gamma-ray burst that occurred when the two stars merged 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 neutron stars.
This magnetar appears to have persisted for more than a day before collapsing into a black hole.
“A neutron star this massive with such a long lifespan is not normally thought to be possible,” astronomer Nuria Jordana-Mitjans of the University of Bath in the UK. Said The Guardian. “It’s a mystery why this one was so long-lived.”
Neutron stars are on a spectrum of how a star can end up at the end of its life. For millions or billions (or potentially trillions) of years, a star will keep going, an engine fusing atoms together in its hot, pressurized core.
Eventually, the atoms that a star can fuse will run out, and at this point, everything will explode. The star expels its outer mass and, no longer supported by the outward pressure provided by fusion, the core collapses under the inward pressure of gravity.
How we classify these collapsed nuclei depends on the mass of the object. The cores of stars that started out with around 8 times the mass of the Sun collapse into white dwarfs, which have an upper mass limit of 1.4 solar masses, compressed into a sphere the size of Earth.
The cores of stars between 8 and 30 solar masses become neutron stars, between around 1.1 and 2.3 solar masses, in a sphere only 20 kilometers (12 miles) in diameter. And the largest stars, above the neutron star’s upper mass limit, collapse into black holes, the theory goes.
But there is a very notable paucity of black holes below 5 solar masses, so what happens in that mass regime is largely a mystery.
This is why neutron star mergers are so interesting to astronomers. They occur when two neutron stars are in a binary system and have reached the orbital decay point where they inevitably come together and become an object that combines the two neutron stars.
Most binary neutron stars have a combined mass that exceeds the theoretical upper mass limit for neutron stars. Therefore, the products of these mergers are likely to sit solidly within that mass gap between the neutron star and the black hole.
When they collide, binary neutron stars release a burst of high-energy radiation known as a short-duration gamma-ray burst. Scientists had thought that these could only be emitted during the formation of a black hole.
But exactly how merged neutron stars become a black hole has been something of a puzzle. Does the black hole form instantly, or do the two neutron stars produce a very heavy neutron star which then collapses into a black hole very quickly, no more than a few hundred milliseconds after the merger?
GRB 180618A was a short-duration gamma-ray burst detected in June 2018, light that 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 outburst itself, the kilonova explosion, and the longer-lasting afterglow.
But when they looked at the electromagnetic radiation produced by the event over time, something was wrong.
The optical emission from the glow disappeared 35 minutes after the gamma-ray burst. The team found that this was because it was expanding at close to the speed of light, accelerated by a continuous power source.
This was not consistent with a black hole, but with a neutron star. And not just any neutron star. It appeared to be what we call a magnetar: one with a magnetic field 1,000 times more powerful than that of an ordinary neutron star, and one quadrillion times more powerful than Earth’s. And it stayed for more than 100,000 seconds (almost 28 hours).
“For the first time,” Jordana-Medios says, “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.”
It’s not clear what might have helped the magnetar live so long. It’s possible that the magnetic field gave it a little help, providing an outward pull that kept it from collapsing completely, at least for a while.
Whatever the mechanism, and this is definitely going to warrant further investigation, the team’s work shows that superamassive neutron stars are capable of launching short-duration gamma-ray bursts, and that we can no longer assume the presence of a black hole.
“Such findings are important as they confirm that newborn neutron stars can power some short-lived GRBs and the bright emissions across the electromagnetic spectrum that have been detected accompanying them.” Jordana-Media says.
“This discovery may offer a new way to locate neutron star mergers, and thus gravitational wave emitters, when we are searching the skies for signals.”
The research has been published in The Astrophysical Journal.