(ORDO NEWS) — The colossal explosion resulting from the merger of two neutron stars has an unexpectedly perfect shape.
According to a new analysis of the consequences of the historic neutron star collision observed in 2017, the kilonova explosion produced by two stars was a completely symmetrical, almost perfect sphere.
And astronomers just don’t know why. This contradicts all previous assumptions and kilonova models.
“No one expected the explosion to look like this. It makes no sense that it is spherical, like a ball,” says astrophysicist Darak Watson from Niels. Bohr Institute in Denmark.
“But our calculations clearly show that this is the case. This likely means that important physics is missing from the kilonova theories and simulations that we have been looking at over the past 25 years.”
We rarely see neutron star collisions. The 2017 explosion, dubbed GW170817, was not only the first ever, it was unsurpassed in terms of detail. From it we learned a number of things about the universe.
For example, these collisions are the source of bursts of gamma rays, the most energetic light in the universe.
The resulting kilonova explosions are also factories for the production of heavy elements such as gold and platinum.
But we still don’t know much about them. Fortunately, so much data has been collected on GW170817 that scientists are still sifting through it all and will be doing so for some time to come.
This led astrophysicist Albert Sneppen of the Niels Bohr Institute and his colleagues to a project to determine the shape of the kilonova.
This is due to the fact that the geometry of the explosion is determined by the properties of superdense matter that make up neutron stars, and could 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 imposing more detailed constraints on known properties.
The spherical explosion they actually detected suggests that our understanding of neutron star mergers is lacking.
“You have two ultra-compact stars that orbit each other 100 times per second before collapsing.
Our intuition, and all previous models, say that the explosive cloud created by the collision should have a flattened and rather asymmetric shape, ”says Snappen.
“The most likely way to make the explosion spherical is if a huge amount of energy escapes from the center of the explosion and flattens out a shape that would otherwise be asymmetric.
Thus, the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unforeseen.”
There’s a possible explanation for this. Neutron stars are what stars of a given mass can turn into after they have used up all the fusion fuel in their core.
When a star reaches this point, it ejects its outer material and the core collapses into a superdense object.
Smaller stars become white dwarfs with about 1.4 times the mass of the Sun.
Intermediate-range stars turn into neutron stars, with a mass about 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 collapse even more gravitationally, turning into a black hole.
But in the short time before this happens, the object can turn into a hypermassive neutron star with an extremely powerful magnetic field.
Recent analysis shows that this is exactly what happened with GW170817. For just a second, the object was a hypermassive neutron star.
This could explain the spherical shape of the kilonova, the researchers say.
“Perhaps at the moment when the energy of the huge magnetic field of a hypermassive neutron star is released as the star collapses into a black hole,” explains Watson.
“The release of magnetic energy can cause matter to be more spherically distributed during the explosion. In this case, the birth of a black hole can be very energetic.”
However, some questions remain unanswered, especially about how heavy elements are forged in the kilonova.
We know this is happening; after the explosion, scientists clearly detected strontium in kilonova emissions.
In their analysis of the kilonova, Snappen’s group found an almost spherically symmetrical distribution of strontium, which is one of the lightest heavy elements.
But the models suggest that heavier elements such as gold and uranium should form at different locations on the kilonova, rather than lighter ones. The team believes this suggests the involvement of neutrinos.
“An alternative idea is that in the milliseconds of a hypermassive neutron star’s life, it emits very powerful radiation, possibly including a huge amount of neutrinos,” Sneppen. speaks.
“Neutrinos can cause neutrons to convert into protons and electrons, and thus create lighter elements in general.
This idea also has flaws, but we believe that neutrinos play an even more important role than we thought. ”
It is possible that more than one mechanism is involved. We hope that the detection of new neutron star collisions in action in the future will help to identify them.
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