Surprisingly spherical neutron explosion was bright as a billion suns

The almost perfectly spherical blast, called a kilonova, led to the birth of a colossal black hole.
Chris Young
Illustration of spherical explosion.
Illustration of spherical explosion.

Albert Sneppen 

Astronomers observed a neutron star merger they described as the "perfect explosion."

The merger of the two incredibly dense stars resulted in a colossal and almost perfectly spherical blast that ultimately resulted in the creation of a black hole.

Scientists described the blast, called a kilonova, for the first time on Wednesday, February 16, after observing it using the European Southern Observatory's (ESO's) Chile-based Very Large Telescope.

The existence of kilonova explosions was first proposed in 1974 and was confirmed in 2013. Only now, though, does the world know what they look like.

Massive kilonova explosion as bright as a billion suns

The scientists behind the observations said the colossal fireball they observed massively exceeded their expectations. The two merging neutron stars, with a combined mass of roughly 2.7 times our sun, orbited each other for billions of years before smashing into each other.

The massive explosion occurred in a galaxy called NGC 4993, roughly 150 million light-years from Earth. The explosion was first detected in 2017 and has been studied in great detail until today.

They calculated that the two merging neutron stars collided at about 25 percent of the speed of light. Their collision created the most intense magnetic fields in the universe. The resulting explosion emitted the luminosity of about a billion suns, according to The Guardian.

"No one expected the explosion to look like this"

Before their observations, the team of researchers expected the explosion might look like a flattened disk. The fact that the explosion looks so different from what was predicted means there may be principles of physics at play that we do not yet understand.

"No one expected the explosion to look like this. It makes no sense that it is spherical, like a ball," second author Darach Watson explained in a press statement. "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."

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The merging neutron stars momentarily formed a single massive neutron star before collapsing into a black hole. The intense densities and temperatures likely created heavy elements, such as gold, platinum, and uranium. The researchers posited that neutrinos or overwhelming magnetic forces might be responsible for the spherical shape of the explosion. However, they also admit that the newly-observed mammoth explosion means astrophysicists the world over will have to go back to the drawing board to try to understand the forces at play.

The study paper was published in the journal Nature.

Study abstract:

The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova1,2,3,4. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta5,6,7. Previously, Sr+ was identified in the spectrum8 of the only well-studied kilonova9,10,11 AT2017gfo12, associated with the gravitational wave event GW170817. Here we combine the strong Sr+ P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source13 also show the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black-hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta; however, an additional process seems necessary to make the element distribution uniform.

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