A flash of light emitted by colliding neutron stars has once again disrupted our understanding of how the Universe works.
Analysis of the brief burst of gamma rays spewed out 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 heavier than the maximum estimated neutron star mass.
This magnetar appears to have persisted for more than a day before collapsing into a black hole.
“It’s not normally thought that such a massive neutron star with a long life expectancy is possible,” astronomer Nuria Jordana-Mitjans of the University of Bath in the UK told The Guardian. “It’s a mystery why this one took so long.”
Neutron stars are on a spectrum of how a star can end up at the end of its life. Over millions or billions (or potentially trillions) of years, a star will pulsate, an engine fusing atoms in its hot, pressurized core.
Eventually, the atoms a star can fuse will run out, and at that point everything explodes. The star ejects its outer mass and, no longer supported by the outward pressure supplied by fusion, the core collapses under the inward pressure of gravity.
How we categorize these collapsed cores depends on the object’s mass. The cores of stars that started out up to 8 times the mass of the Sun collapse into white dwarfs, which have an upper mass limit of 1.4 solar masses, squashed into an Earth-sized sphere.
The cores of stars between 8 and 30 solar masses become neutron stars, between 1.1 and 2.3 solar masses, in a sphere only 20 kilometers (12 miles) in diameter). And the largest stars, above the upper mass limit of neutron stars, collapse into black holes, the theory goes.
But there is a very noticeable dearth of black holes below 5 solar masses, so what happens in this 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 point of orbital decay where they inevitably merge and become an object by combining the two neutron stars.
Most binary neutron stars have a combined mass that exceeds the theoretical upper mass limit for neutron stars. The products of these mergers are therefore likely to sit solidly within this mass gap between neutron star and black hole.
When binary neutron stars collide, they release a burst of high-energy radiation known as a short-lived gamma-ray burst. Scientists had thought that they could only be emitted during the formation of a black hole.
But exactly how merging neutron stars become a black hole has been something of a puzzle. Does the black hole form instantaneously, or do the two neutron stars produce a very heavy neutron star that then collapses into a black hole very quickly, no more than a few hundred milliseconds after the merger?
GRB 180618A was a short-lived gamma-ray burst detected in June 2018, light that had traveled 10.6 billion years to reach us. Jordana-Mitjans and her colleagues wanted to dig deeper into the light emitted by this object: the explosion itself, the kilonova explosion, and the longer glow.
But when they looked at the electromagnetic radiation produced by the event over time, something was off.
The optical emission from the afterglow 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 energy source.
This was not consistent with a black hole, but 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 stronger than that of a normal neutron star and a quadrillion times stronger than Earth’s. And it stayed for over 100,000 seconds (almost 28 hours).
“For the first time,” says Jordana-Mitjans, “our observations highlight multiple signals of a surviving neutron star that lived for at least a day after the original binary neutron star died.”
What might have helped the magnetar live so long is unclear. It’s possible that the magnetic field helped it a bit, providing an outward push that kept it from completely collapsing, at least for a while.
Whatever the mechanism, and this will certainly warrant further investigation, the team’s work shows that supermassive neutron stars are capable of emitting short-lived gamma-ray bursts and that we can no longer assume the presence of a black hole.
“These discoveries are important because they confirm that newborn neutron stars can power some short-lived GRBs and the bright emissions across the electromagnetic spectrum that have been detected to accompany them,” says Jordana-Mitjans.
“This discovery may provide a new way to locate merging neutron stars, and thus gravitational wave emitters, when we look for signals in the sky.”
The research has been published in The Astrophysical Journal.