Flashes of light emitted by colliding neutron stars have further enhanced our understanding of how the Universe works.
Analysis of the short gamma-ray bursts when the two stars merged revealed that, instead of forming a black hole, as expected, the resulting merger was a neutron star with a magnetic core much heavier than the mass of the neutron star.
This magnetism appears to have persisted for a day before collapsing into a black hole.
“Such a massive, long-lived neutron star is not usually thought possible,” astronomer Nuria Jordana-Mitjans of the University of Bath in the UK said. The Guardian. “It’s a mystery why this one lasted so long.”
Neutron stars are on the spectrum of how a star dies at the end of its life. For millions or billions (or perhaps trillions) of years, the star spins around, an engine combining atoms at its core and core.
Eventually, the atoms that the star can hold together run out, and at this point, everything explodes. The star sheds its outer mass and, no longer supported by the outward pressure provided by fusion, its core collapses under the inward pressure of gravity.
How we distribute the cores depends on the size of the product. Stars that started out with 8 times the mass of the Sun fall down to be white, which has a maximum limit of 1.4 solar mass, squilled into a sphere about the size of the Earth.
Stars between 8 and 30 solar masses transform into neutron stars, between 1.1 and 2.3 solar masses, in a region 20 kilometers (12 miles) across). And the most massive stars, beyond the size of a neutron star, fall into black holes, according to theory.
But there is a significant decrease in black holes below 5 solar power, so what happens in this large order is not understood.
This is why neutron star mergers are of great interest to astronomers. It occurs when two neutron stars are in a binary system and have reached the point of orbital decay when they inevitably collide and become a single object combining two neutron stars.
Most neutron stars have a combined mass that exceeds the theoretical limit for neutron stars. Therefore these fusion products can be solid within the neutron star-black hole mass gap.
When two neutron stars collide, they emit a burst of energy known as a short-lived gamma-ray burst. Scientists thought that this could be released only by creating a black hole.
But exactly how a neutron star merges into a black hole remains a mystery. Does the black hole form instantaneously, or do two neutron stars produce a supermassive neutron star that collapses into the black hole very quickly, no more than a few hundred milliseconds after merging?
GRB 180618A was a short burst discovered in June 2018, light that traveled 10.6 billion years to reach us. Jordana-Mitjans and his colleagues wanted to observe the light emitted by this object: the explosion itself, the kilonova explosion, and the long-lived light.
But, when he looked at the electromagnetic radiation produced by the event over time, something was off.
The light faded 35 minutes after the gamma-ray burst. This, the team found, was because it was growing close to the speed of light, accelerated by a continuous energy source.
This was not associated with a black hole, but with a neutron star. Not just any neutron star. It looked like what we call a magnetosphere: one with a magnetic field 1,000 times that of a typical neutron star, and twice as strong as Earth. And it stayed around for over 100,000 seconds (about 28 hours).
Jordana-Mitjans says: “For the first time, our observations show several signals from a neutron star remnant that existed for one day after the death of the original neutron star binary.”
What would have enabled the magnet to survive for such a long time is unknown. It is possible that the magnetic force provided a little help, providing an external pull that prevented the fall all the way, for a short time.
Whatever the mechanism was – and this will lead to further research – the team’s work shows that supermassive neutron stars can cause short-term explosions, and that we can no longer think of a black hole.
“These findings are important because they confirm that newborn neutron stars can be responsible for short-lived GRBs and the bright gas across the electromagnetic spectrum that has been found to accompany them,” says Jordana-Mitjans.
“This discovery could provide a new way to find neutron stars, and thus the gravitational waves they emit, as we search the atmosphere for signs.”
Research has been published in The Astrophysical Journal.