Researchers used the Fugaku supercomputer in Japan to simulate the merger of neutron stars – from spinning to black hole formation and gamma-ray burst – and decipher how heavy elements like gold are formed.
Deciphering the secrets of neutron star mergers
When two neutron stars collide, the universe sends out a spectacular series of signals – gravitational waves, flashes of light, neutron streams and bursts of energy across the entire electromagnetic spectrum. These rare cosmic collisions are perfect candidates for multi-messenger astronomy, a powerful new way to observe the universe that combines information from different types of detectors to get a complete picture of the event.
To pick up all these signals, scientists don’t rely on traditional telescopes. They rely on a global network of instruments that includes gravitational wave detectors, neutrino observatories, and space and ground-based telescopes. But coordinating all of these requires very precise models to know exactly what to look for—and when.
This is where an important breakthrough comes into play.
"It is very difficult to predict the multiple messenger signals from binary neutrino star mergers from first principles. Now we have been able to do just that," says Kota Hayashi of the Max Planck Institute. "Using the FUGCO supercomputer in Japan, we have performed the longest and most complex simulation to date of a binary neutrino star merger."
Hayashi and his team used Japan’s Fugaku supercomputer, one of the world’s most powerful, to simulate a neutrino star collision from start to finish. The simulation, the longest and most detailed ever performed, involved a second and a half of real-time and used 130 million processor hours. At its peak, it ran on up to 80,000 processors simultaneously.
The model includes the effects of Einstein's general theory of relativity, neutrino emissions and strong magnetic fields, and captures the extreme physics inside these dying, dense stars as they spiral together, collide and form a black hole.
One and a half seconds, 130 million CPU hours
The simulation starts with very few assumptions – neutron stars with strong magnetic fields orbiting each other – and evolves the binary system self-consistently over time based on fundamental physical principles. “Our new simulation follows the binary system through its entire evolution: spinning, merging, and the post-merger phase, including the formation of the jet. It provides the first complete picture of the entire process and valuable information for future observations of such events,” explains Kota Hayashi.
Initially, the two neutron stars (simulated to have masses of 1.25 and 1.65 times the mass of the Sun) orbit each other five times. During this spinning phase, they fall into each other as they lose orbital energy, which is emitted as gravitational waves. Because of the large total mass, the remnant of the merger immediately collapses into a black hole. The simulation predicts the gravitational wave signal, the first of the multiple messenger signals that can be observed.
Magnetic turmoil and energy jets
After the merger, a disk of material forms around the remnant of the black hole. In the disk, the winding of the field lines and the dynamo effect increase the magnetic field. The interaction with the black hole's rapid rotation further strengthens the magnetic field. This creates an outward flow of energy along the black hole's axis of rotation.
"We think that this flow of energy along the axis of the black hole, caused by magnetic fields, drives a gamma-ray burst," says researcher Masaru Shibata. "This fits with what we know from previous observations and provides further insights into the internal mechanism of neutron star mergers."
The team also used their simulation to derive the expected neutron emission from binary neutron star mergers. “What we have learned about jet generation and the dynamics of magnetic fields is critical to our interpretation and understanding of neutron star mergers and their associated counterparts,” explains Masaru Shibata. The simulation provides information about the amount of material ejected into the interstellar medium, thereby allowing us to predict the kilonova. This is the luminous cloud of gas and dust that is rich in heavy elements. When the first collision between two neutron stars on August 17, 2017, was detected and monitored by gravitational wave detectors and later by various other telescopes, the researchers detected elements heavier than iron, especially gold. Although theoretical physicists had suspected that such kilonovae produce these extra-heavy elements, this theory was first confirmed in 2017. Only iron and lighter elements can form in the interior of stars.
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