Scientists have discovered a new way to study the mysterious interiors of neutron stars by listening to the “pure tone” they produce after a collision.
A team from Goethe University Frankfurt found that this signal, known as the “long ringdown,” could provide important clues about what neutron stars are made of.
Their research was recently published in Nature Communications.
Neutron stars are incredibly dense objects—just a few kilometers wide but packed with more mass than our Sun.
Because their interiors are under extreme pressure, scientists don’t fully understand what they are made of.
However, when two neutron stars collide, they create a powerful explosion that sends out gravitational waves—ripples in space and time.
These waves were first detected in 2017, but the most valuable signals happen just after the moment of impact. As the newly formed, rapidly spinning remnant settles down, it vibrates like a struck tuning fork, producing a “pure tone” that scientists call the long ringdown.
The team at Goethe University found that this long ringdown signal is closely linked to the “equation of state”—a key formula that describes how matter behaves at extreme densities.
Different neutron stars, depending on their composition, will produce slightly different frequencies of this pure tone, much like how tuning forks made of different materials create different sounds.
“Just like a tuning fork, the remnants of a neutron star collision will ring at a specific frequency based on what they are made of,” explains Professor Luciano Rezzolla, one of the study’s authors. “By measuring this frequency, we can learn about the mysterious cores of neutron stars.”
To test their theory, the researchers ran advanced computer simulations using different models of neutron-star matter.
They found that analyzing the long ringdown could greatly reduce uncertainty about the equation of state, helping scientists understand matter under extreme conditions.
Dr. Christian Ecker, the study’s first author, highlights the significance of the discovery: “Thanks to advanced simulations, we identified this new phase in the merger process. It could provide a much clearer picture of what happens inside neutron stars.”
Co-author Dr. Tyler Gorda adds that their approach was also efficient: “By carefully choosing a few key models, we could get the same insights as a much larger set of simulations. This not only saves computer time and energy but also strengthens our confidence in the results.”
So far, gravitational-wave detectors have not been able to capture the post-merger signal, but future observatories—such as the Einstein Telescope, expected to be operational within the next decade—might change that.
When these next-generation detectors come online, the long ringdown could become a powerful tool for unlocking the deepest secrets of neutron stars.
Source: KSR.
