Scientists have investigated the gravitational waves born from the merger of neutron stars. It turned out that as these oscillations faded, they came to a single tone similar to that generated by the tuning fork.
Waves from neutron star mergers
Scientists from Goethe University Frankfurt have found a new way to study the interior of neutron stars using gravitational waves from their collisions. By analyzing the “long fading” phase, the pure tone signal emitted by the post-merger remnant, they found a strong correlation between the properties of the signal and the neutron star’s equation of state of matter. Their findings were published in the journal Nature Communications.
Neutron stars, whose mass exceeds a mass of the entire Solar System, bounded by an almost perfect sphere with a diameter of only a few tens of kilometers, are among the most interesting astrophysical objects known to mankind. However, the extreme conditions in their interior make their composition and structure highly uncertain.
A collision between two neutron stars, like the one we observed in 2017, offers a unique opportunity to unlock these mysteries. As binary neutron stars spiral for millions of years, they emit gravitational waves, but the most intense emission occurs at the moment of merger and a few milliseconds afterward.
“Pure Tone” after the merger
The post-merger remnant, a massive object spinning rapidly as a result of the collision, emits gravitational waves in a powerful but narrow range of frequencies. This signal contains important information about the so-called “equation of state” of nuclear matter, which describes how matter behaves at extreme densities and pressures.
The group of Prof. Luciano Rezzolla from Goethe University Frankfurt found that although the amplitude of the gravitational-wave signal after fusion decreases with time, it becomes more and more “pure” — tends to a single frequency, like a giant tuning fork resonating after a blow. They called this phase “long ringdown” and found a close connection between its unique characteristics and the properties of the densest regions in the cores of neutron stars.
“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,” Rezzolla says.
Long compression phase of a neutron star
Using current general-relativistic neutron star merger simulations with carefully constructed equations of state, the researchers demonstrated that long collapse analysis can significantly reduce the uncertainty in the equation of state at very high densities — where there are no direct constraints.
“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.”
Although current gravitational wave detectors have not observed a post-merger signal yet, scientists are optimistic that next-generation detectors such as the Einstein telescope, which is expected to begin operating in Europe within the next decade, will make this long-awaited detection possible. When this happens, the long flare will be a powerful tool for exploring the mysterious insides of neutron stars and unlocking the secrets of matter at its most extreme.
According to phys.org
