Neutron Stars Collide and Ring Like a Cosmic Bell, Revealing Hidden Secrets

When neutron stars collide, they create a powerful gravitational-wave signal, with the post-merger remnant ringing like a cosmic tuning fork.

Scientists have identified this phase, the “long ringdown,” as a key to understanding the extreme matter inside neutron stars.

Neutron Stars and Their Mysterious Interiors

Neutron stars are some of the most extreme objects in the universe. Despite being just a dozen kilometers across, they pack more mass than our entire solar system. Their interiors are incredibly dense and mysterious, making it difficult for scientists to fully understand their composition and structure.

However, when two neutron stars collide — like the famous merger observed in 2017 — they create a unique opportunity to study these mysteries. As they spiral toward each other over millions of years, they emit gravitational waves, but the most intense signals occur in the final moments of merging and just afterward. The aftermath of the collision forms a massive, rapidly spinning remnant that continues to emit gravitational waves within a narrow frequency range. This signal carries valuable clues about the “equation of state” of nuclear matter, which governs how matter behaves under extreme pressure and density.

The “Long Ringdown” Phenomenon

A research team led by Prof. Luciano Rezzolla at Goethe University Frankfurt has made a key discovery about these post-merger signals. While the gravitational waves gradually weaken, they become more refined over time, settling into a single dominant frequency — much like a tuning fork ringing after being struck. The team has named this phase the “long ringdown” and found that its characteristics are directly linked to the properties of the densest regions inside neutron stars.

A New Way to Probe the Densest Matter

“Just like tuning forks of different material will have different pure tones, remnants described by different equations of state will ring down at different frequencies. The detection of this signal thus has the potential to reveal what neutron stars are made of,” says Rezzolla, adding, “I am particularly proud of this work as it constitutes exemplary evidence of the excellence of Frankfurt- and Darmstadt-based scientists in the study of neutron stars, which have been a central focus of the Hessian research cluster ELEMENTS.”

High-Precision Simulations Unlock New Insights

Using advanced general-relativistic simulations of merging neutron stars with carefully constructed equations of state, the researchers demonstrated that analyzing the long ringdown can significantly reduce uncertainties in the equation of state at very high densities – where no direct constraints are currently available. “Thanks to advances in statistical modeling and high-precision simulations on Germany’s most powerful supercomputers, we have discovered a new phase of the long ringdown in neutron star mergers,” says Dr. Christian Ecker, first author of the study, “It has the potential to provide new and stringent constraints on the state of matter in neutron stars. This finding paves the way for a better understanding of dense neutron star matter, especially as new events are observed in the future.”

Co-author Dr. Tyler Gorda adds: “By cleverly selecting a few equations of state, we were able to effectively simulate the results of a full statistical ensemble of matter models with considerably less effort. Not only does this result in less computer time and energy consumption, but it also gives us confidence that our results are robust and will be applicable to whatever equation of state actually occurs in nature.”

Future Detectors and the Path Forward

While current gravitational-wave detectors have not yet observed the post-merger signal, scientists are optimistic that the next-generation detectors, such as the Einstein Telescope expected to become operational in Europe within the next decade, will make this long-awaited detection possible. When that happens, the long ringdown will serve as a powerful tool to probe the enigmatic interiors of neutron stars and reveal the secrets of matter at its most extreme.

Reference: “Constraining the equation of state in neutron-star cores via the long-ringdown signal” by Christian Ecker, Tyler Gorda, Aleksi Kurkela and Luciano Rezzolla, 3 February 2025, Nature Communications.
DOI: 10.1038/s41467-025-56500-x

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