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Supercomputer simulation reveals how merging neutron stars form black holes and powerful jets

Merging neutron stars are excellent targets for multi-messenger astronomy. This modern and still very young method of astrophysics coordinates observations of the various signals from one and the same astrophysical source. When two neutron stars collide, they emit gravitational waves, neutrinos and radiation across the entire electromagnetic spectrum. To detect them, researchers need to add gravitational wave detectors and neutrino telescopes to ordinary telescopes that capture light.

Precise models and predictions of the expected signals are essential in order to coordinate these observatories, which are very different in nature.

"Predicting the multi-messenger signals from binary neutron star mergers from first principles is extremely difficult. We have now succeeded in doing just that," says Kota Hayashi, a postdoctoral researcher in the Computational Relativistic Astrophysics department at the Max Planck Institute for Gravitational Â鶹ÒùÔºics (Albert Einstein Institute) in the Potsdam Science Park. "Using the Fugaku supercomputer in Japan, we have performed the longest and most complex simulation of a binary neutron star merger to date."

The simulation spans 1.5 seconds of real time, took 130 million CPU hours to complete, and kept between 20,000 and 80,000 CPUs busy at any one time. It includes the effects of Einstein's theory of general relativity, neutrino emission, and the interaction of strong magnetic fields with the high-density matter inside the merging neutron stars.

The research has been accepted by Â鶹ÒùÔºical Review Letters and is currently on the arXiv preprint server.

A complete picture

The simulation starts with very few assumptions—neutron stars with strong magnetic fields orbiting each other—and evolves the binary self-consistently over time based on basic physical principles.

"Our new simulation follows the binary all the way through its evolution: inspiral, merger, and the post-merger phase, including the jet formation. It provides the first complete picture of the entire process and thus valuable information for future observations of such events," explains Hayashi.

Initially, the two neutron stars (simulated at 1.25 and 1.65 times the mass of our sun) orbit each other five times. During this inspiral phase, they fall towards each other as they lose orbital energy, which is emitted as gravitational waves. Because of the high total mass, the merger remnant promptly collapses into a black hole. The simulation predicts the gravitational-wave signal, the first of the observable multi-messenger signals.

After the merger, a disk of matter forms around the remnant black hole. In the disk, the magnetic field is amplified by the winding of the field lines and dynamo effects. The interaction with the fast spin of the black hole then further intensifies the magnetic field. This creates an outflow of energy along the rotational axis of the black hole.

"We think that this energy flow along the black hole axis, driven by magnetic fields, powers a gamma-ray burst," says Masaru Shibata, director of the Computational Relativistic Astrophysics department. "This agrees with what we know from previous observations and provides further insight into the inner workings of neutron star mergers."

Multi-messenger predictions

The team further uses their simulation to derive the expected emission of neutrinos from binary neutron star mergers.

"What we've learned about jet formation and magnetic field dynamics is crucial for our interpretation and understanding of neutron star mergers and their associated counterparts," explains Shibata. The simulation provides information on how much matter is ejected into the interstellar medium and thus makes it possible to predict the kilonova. This is the luminous cloud of gas and dust that is rich in heavy elements.

When the first collision of two neutron stars on 17 August 2017 was detected and monitored by gravitational wave detectors and subsequently by various other telescopes, researchers discovered elements such as gold in particular, which are heavier than iron. Even though theoretical physicists suspected such kilonovae of producing these particularly heavy elements, this theory was confirmed for the first time in 2017. Only iron and lighter elements can be created in the interior of stars.