When neutron stars collide, they produce powerful gravitational wave signals, and the remnants of the merger can sound like a cosmic tuning fork. Scientists have identified this phase, known as "long ringing," as key to understanding the extreme matter inside neutron stars.
Neutron stars and their mysterious interiors
Neutron stars are the most extreme objects in the universe. Although they are only a dozen kilometers in diameter, their mass exceeds that of our entire solar system. Their interiors are very dense and mysterious, making it difficult for scientists to fully understand their composition and structure.
However, when two neutron stars collide - such as the famous merger observed in 2017 - they create a unique opportunity to study these mysteries. Over millions of years, as they spiraled into each other, they emitted gravitational waves, but the strongest signal occurred during the final moments of the merger and afterward. The aftermath of the collision created a large, rapidly spinning remnant that continues to emit gravitational waves in a narrow frequency range. This signal contains valuable clues about the "equation of state" of nuclear matter, which determines how matter behaves under extreme pressures and densities.
"Long Ringing" Phenomenon
A research team led by Professor Luciano Rezzola of Goethe University Frankfurt made an important discovery about these combined signals. While gravitational waves weaken, they become more refined over time, settling on a single dominant frequency - like the chime a tuning fork makes when struck. The team named this phase "long ringing" and found that its characteristics are directly related to the properties of the densest regions inside the neutron star.
New way to detect the densest matter
"Just like tuning forks of different materials emit different pure tones, remnants described by different equations of state emit sounds at different frequencies. The detection of this signal therefore has the potential to reveal what neutron stars are made of," Rezzola said. "I am particularly proud of this work because it is an example of the excellence of the scientists in Frankfurt and Darmstadt in the field of neutron star research, which has always been a core focus of the Hessian research cluster ELEMENTS."
High-precision simulation reveals new insights
Using carefully constructed equations of state, the researchers performed advanced general relativity simulations of merging neutron stars and showed that analyzing long circulations can significantly reduce the uncertainty in the equations of state at very high densities - where there are currently no direct constraints. Dr. Christian Ecker, first author of the study, said: "Thanks to advances in statistical modeling and high-precision simulations on Germany's most powerful supercomputers, we have discovered a new phase of long circulation in neutron star mergers. This discovery paves the way for a better understanding of dense neutron star matter, especially if new events are observed in the future."
Co-author Dr. Tyler Gorda added: "By cleverly choosing a few equations of state, we were able to efficiently simulate the results of an entire statistical combination of material models with significantly less effort. Not only did this reduce computer time and energy consumption, it also gave us confidence that our results were robust and would apply to any equation of state that actually occurs in nature."
Detectors of the future and the way forward
While current gravitational wave detectors have not yet observed the merged signal, scientists are optimistic that next-generation detectors, such as the Einstein Telescope expected to be operational in Europe within the next decade, will make this long-awaited detection possible. By then, long circulation will become a powerful tool for probing the mysterious interior of neutron stars and reveal the most extreme secrets of matter.
Compiled from /ScitechDaily
DOI:10.1038/s41467-025-56500-x