When neutron stars collide, they produce a powerful explosion. The first collision that astronomers have clearly observed has shown that, contrary to expectations, the explosion was perfectly spherical. The formation of the spherical shape is still a mystery to researchers, but the discovery could open new doors in fundamental physics and the determination of the age of the universe.
Researchers from the University of Turku were part of an international collaboration led by astrophysicists at the University of Copenhagen, which resulted in the discovery. The study has just been published in the journal Nature.
Kilonovae are massive explosions that occur when two neutron stars orbiting each other finally collide. Kilonovae are responsible for creating both large and small things in the universe, from black holes to atoms in a gold ring and the iodine in our bodies. They create the most extreme physical conditions in our universe, and it is in these conditions that the universe creates the heaviest elements of the periodic table, such as gold, platinum, and uranium.
However, there is still much that is unknown about this extremely powerful phenomenon. The detection of the kilonova discovered in 2017, which was 140 million light-years away, was the first time that researchers were able to gather information about kilonovae using ground- and space-based telescopes. Scientists around the world are still interpreting the data from this massive explosion.
"There are two super-compact stars that orbit each other 100 times per second before collapsing. Our intuition and all previous models suggest that the explosion generated by the collision should be flattened and quite asymmetric," says Albert Sneppen, the lead author of the study and a PhD student at the Niels Bohr Institute at the University of Copenhagen.
For this reason, Sneppen and his colleagues were surprised to find that this was not the case for the kilonova in 2017. The explosion was completely symmetrical and almost perfectly round.
"It's amazing and completely counterintuitive to observe this kind of explosion, which is as round as a ball. Our calculations, however, clearly show that it is," says Rubina Kotak, a research fellow at the University of Turku.
The round shape of the kilonova puzzles researchers. The explosion involves unexpected physics, according to the scientists.
"A round explosion probably occurs because a massive amount of energy is released from the center of the explosion, smoothing out an otherwise asymmetrical shape. So, the spherical shape is a sign that there is likely much more energy than expected at the center of the collision," says Sneppen.
When neutron stars collide, they merge into one hypermassive neutron star, which collapses into a black hole. Researchers wonder if the secret to the spherical shape lies in this collapse. "Perhaps a kind of 'magnetic bomb' is created at that moment when energy is released from the hypermassive neutron star's enormous magnetic field as the star collapses into a black hole. The release of magnetic energy could cause the explosion material to be distributed more spherically. In that case, the creation of a black hole could be very energetic," says Darach Watson, an associate professor at the Niels Bohr Institute.
"At present, no single theory can explain all the observational results. We look forward to new observations of kilonovae and their shapes in the coming years. Projects such as the Gravitational-wave Optical Transient Observer (GOTO), in which the University of Turku is a partner, are crucial in the detection of kilonovae and similar objects," says Kotak.
A new cosmic ruler
The shape of the explosion is also interesting for a completely different reason.
Astronomers are debating the expansion rate of the universe. The expansion rate tells us how fast the universe is growing, and therefore, how old it is. Two existing methods of measuring the expansion rate differ by about a billion years. In the perfectly round explosion of the kilonova, however, astronomers may have a third method to help settle the debate.
Previously, supernovae were used to measure distances in the universe, but kilonovae could provide a cleaner and more accurate method. The shape of the kilonova explosion is crucial because if the object is not perfectly round, it will emit radiation differently depending on the viewing angle.
Although the round shape of the explosion is an exciting discovery, scientists are still unsure how it was formed. The theory is that a massive amount of energy was released from the center of the explosion, smoothing out the asymmetric shape. This suggests that there is more energy in the center of the explosion than anticipated.
Rubina Kotak, an astrophysicist at the University of Turku, stated that the Gravitational-wave Optical Transient Observer (GOTO) project, in which Turku University is a partner, is vital in detecting kilonovae and similar phenomena. She also said that they are eagerly awaiting new observations of kilonovae in the coming years.
The discovery of the perfectly round kilonova has opened up new avenues for basic physics and determining the age of the universe. As the scientific community continues to research the phenomenon, the possibilities and implications of this discovery may be far-reaching.