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Astrophysicists have proposed looking into the interior of neutron stars by recording the oscillations of gravitational waves that occur directly at the moment of merger. The most important information about nuclear processes inside the stars can be revealed by catching a special "clean" signal. Being one of the consequences of stellar evolution, neutron stars are extreme objects: their mass exceeds the solar mass sometimes by 2,16 times, and the radius reaches 10-13 kilometers. Since the pressure and density of matter inside these celestial bodies are so great that they cannot be recreated in laboratory conditions, the question of what role nuclear interactions play at such high pressures remains one of the main ones in modern astrophysics.
That's why scientists are so interested in neutron star mergers (which form a more massive neutron star or black hole). When two neutron stars orbit each other, they spiral out of control due to the emission of gravitational waves—distortions in space-time that travel through the universe at the speed of light.
The most powerful oscillations occur directly at the moment of the merger and in the following milliseconds, when an extremely massive, rapidly rotating neutron star is formed, which for some time emits gravitational waves with a characteristic narrow frequency band. It is this "clean" signal that scientists "hunt". For the first time, the signal emitted by neutron stars during the merger was recorded in 2017 using the gravitational wave detectors LIGO (located in the USA) and Virgo (Italy). The source of the wave, called GW170817, was two objects with a mass of about 1,1 and 1,6 solar masses, respectively.
Now, scientists at the Johann Wolfgang Goethe University in Frankfurt, Germany, have found that while the strength of such a “pure” signal gradually weakens, its frequency tends to some constant value. This stage in astrophysics is called “long-term decay” by analogy with a tuning fork - when the latter eventually loses excess overtones, only one fundamental note remains. The results of the study, published in the journal Nature Communications, showed that it is in this "clean note" that important information about the density and pressure inside neutron stars is hidden.
Scientists came to this conclusion using computer simulations: the "long extinction" turned out to be associated with the maximum possible pressure and density in the cores of stars. Thus, astrophysicists were able to "feel" the most extreme areas of the phase diagram of matter.
"Thanks to the latest statistical modeling methods and high-precision simulations on powerful supercomputers, we have found a new phase of 'long-term quenching' in neutron star mergers. This effect can provide new and strict constraints on the state of matter inside stars," explained one of the authors of the scientific work, Christian Ecker.
Although current gravitational wave observatories (LIGO, Virgo, KAGRA) have not yet been able to hear a clear signal from a neutron star merger, researchers hope that future-generation detectors will be able to register it. If their conclusions are correct, the "long decay" will become an indispensable tool for studying the structure of some of the densest and most extreme objects in the Universe.
