The colossal explosion resulting from a merger between two neutron stars has an unexpectedly perfect shape.
According to a new analysis of the aftermath of a historical neutron star collision observed in 2017, the kilonova explosion produced by the two stars was a completely symmetrical, almost perfect sphere. And astronomers just don't know why. It contradicts all previous assumptions about and models of kilonovae.
"No one expected the explosion to look like this. It makes no sense that it is spherical, like a ball," says astrophysicist Darach Watson of the Niels Bohr Institute in Denmark.
"But our calculations clearly show that it is. This probably means that the theories and simulations of kilonovae that we have been considering over the past 25 years lack important physics."
We rarely see neutron star collisions. That 2017 explosion, named GW170817, wasn't just the first on record, it's remained unbeaten as far as detail goes. From it we learnt a number of things about the Universe. For example, these collisions are a source of bursts of gamma radiation, the most energetic light in the Universe. The resulting kilonova explosions are also factories for producing heavy elements such as gold and platinum.
But there's a lot about them we still don't know. Luckily, there was so much data collected from GW170817 that scientists are still sifting through it all, and will be for some time. This led astrophysicist Albert Sneppen of the Niels Bohr Institute and his colleagues on a project to characterize the shape of the kilonova.
This is because the geometry of the explosion is dictated by the properties of the ultra-dense matter of which neutron stars consist, and can help scientists better understand the energy of the explosion and other properties of the merger.
They thought they knew roughly what they were going to find, and that their work would be about placing more detailed constraints on the known properties. The spherical explosion they actually found suggests that our understanding of neutron star mergers is lacking.
"You have two super-compact stars that orbit each other 100 times a second before collapsing. Our intuition, and all previous models, say that the explosion cloud created by the collision must have a flattened and rather asymmetrical shape," Sneppen says.
"The most likely way to make the explosion spherical is if a huge amount of energy blows out from the center of the explosion and smooths out a shape that would otherwise be asymmetrical. So the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unforeseen."
There is a possible explanation for this. Neutron stars are what stars of a given mass can transform into after they've used all of the fusion fuel in its core. When a star reaches this point, it ejects its outer material and the core collapses into an ultra-dense object.
Smaller stars become white dwarfs, up to around 1.4 times the mass of the Sun. Mid-range stars turn into neutron stars, up to around 2.4 times the mass of the Sun. And more massive stars turn into black holes.
When two neutron stars collide, the combined mass causes the newly formed object to gravitationally collapse further, turning into a black hole. But, for a short period of time before this happens, the object can become a hypermassive neutron star with an extremely powerful magnetic field. Recent analysis suggests that this is what happened with GW170817. For just a second, the object was a hypermassive neutron star.
This could explain the spherical kilonova, the researchers say.
"Perhaps a kind of 'magnetic bomb' is created at the moment when the energy from the hypermassive neutron star's enormous magnetic field is released when the star collapses into a black hole," Watson explains.
"The release of magnetic energy could cause the matter in the explosion to be distributed more spherically. In that case, the birth of the black hole may be very energetic."
But there remain some questions unanswered, specifically about how heavy elements are forged in the kilonova. We know it happens; following the explosion, scientists made a clear detection of strontium in the kilonova ejecta.
In their analysis of the kilonova, Sneppen's team found a nearly spherically symmetric distribution of strontium, which is among the lighter of the heavy elements. But models suggest heavier elements such as gold and uranium should form at separate places in the kilonova from the lighter ones. This, the team believes, suggests that neutrinos are involved.
"An alternative idea is that in the milliseconds that the hypermassive neutron star lives, it emits very powerfully, possibly including a huge number of neutrinos," Sneppen says.
"Neutrinos can cause neutrons to convert into protons and electrons, and thus create more lighter elements overall. This idea also has shortcomings, but we believe that neutrinos play an even more important role than we thought."
It's possible that there could be more than one mechanism at play. Hopefully, catching more neutron star collisions in action in the future could help reveal them.
The research has been published in Nature.