Surprise! Colliding neutron stars create perfectly spherical 'kilonova' explosions

Massive cosmic explosions created when neutron stars collide aren't as chaotic as you might expect.

When neutron stars collide, the explosions they create are perfectly spherical, a new study finds.

This contradicts previous theories surrounding the blasts, known as kilonovas, that suggest they should proceed as flattened discs. But the reason these blasts take a spherical shape is still shrouded in mystery.

Kilonovas are important to our understanding of cosmic evolution because it is in the extreme conditions created by these massive cosmic explosions that heavy elements such as gold, platinum and uranium are synthesized. 

The ultimate result of a kilonova is a so-called "hypermassive" merged neutron star that rapidly collapses to birth a black hole. But other details about these events remain largely unknown, so any information about the collisions that cause them are metaphorical gold dust to astrophysicists.

The first time a kilonova was detected was in 2017, and the cosmic explosion was located around 140 million light-years from Earth. It was while analyzing data from this massive explosion that astrophysicists made the surprise discovery that kilonovas are spherical. 

"No one expected the explosion to look like this. It makes no sense that it is spherical, like a ball. But our calculations clearly show that it is," study coauthor Darach Watson, an associate professor at the Niels Bohr Institute in Copenhagen, said in a statement. 

Study lead author Albert Sneppen, a Ph.D. student at the  Niels Bohr Institute, explained why the discovery of a spherical shape for the 2017 kilonova was so unexpected. "You have two super-compact stars that orbit each other 100 times a second before collapsing," he explained. "Our intuition, and all previous models, say that the explosion cloud created by the collision must have a flattened and rather asymmetrical shape."

The spherical shape of the kilonova indicates to the researchers that there may be hitherto unexpected physics at play when two neutron stars spiral together and merge. 

"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," Sneppen said. "So the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unforeseen."

The team thinks that the secret to the spherical shape of the kilonova might be hidden in the brief existence of the hypermassive neutron star created by the merger and its rapid collapse to a black hole.

"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 said. "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."

While this theory may explain the spherical shape of the kilonova, it fails to account for another unexpected feature spotted by the astrophysicists. 

How do kilonovas "spread the wealth?"

Previous models of kilonovas had suggested that all the elements they forge should be heavier than iron. The extremely heavy elements like gold or uranium should be created in different places in the kilonova than the relatively lighter elements like strontium or krypton. These lighter and heavier elements should also be launched through space in different directions by the massive explosion.

But when looking at the 2017 kilonova, the team found only the lighter elements and also observed that they were distributed evenly throughout space. The researchers believe that neutrinos, ghostly fundamental particles that only weakly interact with matter, could be responsible for this unexpected aspect of their observations. 

"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 said. "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."

The discovery that kilonova explosions are spherical could also help shed light on dark energy, the mysterious force that accounts for around 70% of the cosmos' total energy-matter content and apparently drives the accelerating expansion of the universe. 

Currently, there is a major disparity between measurements of the speed of universal expansion made by observing distant supernovas, cosmic explosions that happen when stars die, and the predictions of that speed made in particle physics.

An illustration of a kilonova and a gamma-ray burst, with blue representing squeezed material and red indicating material ejected by the two neutron stars swirling around the merged object they created.  (Image credit: Aaron M. Geller/Northwestern/CIERA and IT Research Computing Services)

"Among astrophysicists, there is a great deal of discussion about how fast the universe is expanding. The speed tells us, among other things, how old the universe is," Sneppen explained. "And the two methods that exist to measure it disagree by about a billion years. Here we may have a third method that can complement and be tested against the other measurements."

Knowing the shape of the kilonova is vital to turning these cosmic events into a measuring stick. That's because an object that is non-spherical emits light at different orientations based on what angle it is viewed from, whereas a spherical explosion grants a more uniform emission regardless of orientation. This could result in much greater precision when measuring cosmic distances and thus inferring the expansion speed of the universe and its rate of acceleration. 

The study team said that, before kilonovas can be used as measurement tools in this manner, remaining questions raised by this discovery must be answered — meaning more observations of neutron star mergers are needed. 

They hope that the continued work of gravitational wave observatories like the Laser Interferometer Gravitational-Wave Observatory, or LIGO for short, which track the tiny ripples in the fabric of space these mergers launch will allow these kilonova observations to be made. 

The new study was published online Feb. 15 in the journal Nature.