Imagine taking a star twice as massive as the sun and crushing it to the size of Manhattan. The result would be a neutron star — one of the densest objects to be found anywhere in the universe, exceeding the density of any material naturally found on Earth by a factor of tens of trillions. Neutron stars are extraordinary astrophysical objects in their own right, but their extreme densities could also enable them to function as laboratories for studying fundamental questions of nuclear physics, under conditions that could never be reproduced on Earth.
Because of these exotic conditions, scientists still don’t understand what exactly neutron stars themselves are made of, their so-called “equation of state” (EoS). Determining this is an important goal of modern astrophysics research. A new piece of the puzzle, limiting the range of possibilities, has been discovered by a few scientists at IAS: Carolyn Raithel, John N. Bahcall Fellow in the School of Natural Sciences; and Elias Most, a member of the school and John A. Wheeler Fellow at Princeton University. Their work has recently been published in The astrophysical diary letters.
Ideally, scientists would like to look inside these exotic objects, but they are too small and too far away to be imaged with standard telescopes. Scientists instead rely on indirect properties they can measure — such as the mass and radius of a neutron star — to calculate the EoS, in the same way one might use the length of two sides of a right triangle to calculate the hypotenuse. . However, the radius of a neutron star is very difficult to measure accurately. A promising alternative for future observations is to use a quantity called the “peak spectral frequency” (or f.) instead.2) in place.
But how is it going2 measured? Collisions between neutron stars, governed by the laws of Einstein’s theory of relativity, lead to strong bursts of gravitational wave emission. In 2017, scientists measured such emissions directly for the first time. “In principle, the maximum spectral frequency can be calculated from the gravitational wave signal emitted by the wobbling remnant of two neutron stars merged,” Most says.
It was previously expected that f2 would be a reasonable measure of radius, as researchers until now believed that a direct or “quasi-universal” correspondence existed between them. However, Raithel and Most have shown that this is not always true. They have shown that determining the EoS is not the same as solving a simple hypotenuse problem. Instead, it’s more like calculating the longest side of an irregular triangle, which also requires a third piece of information: the angle between the two shorter sides. For Raithel and Most, this third piece of information is the “slope of the mass-beam relationship,” which encodes information about the EoS at higher densities (and thus more extreme conditions) than the radius alone.
This new finding allows researchers working with next-generation gravitational wave observatories (the successors to the currently operating LIGO) to better utilize the data obtained after neutron star mergers. According to Raithel, this data could reveal the fundamental constituents of the matter of neutron stars. “Some theoretical predictions suggest that within the cores of neutron stars, phase transitions could dissolve the neutrons into subatomic particles called quarks,” Raithel said. “This would mean that the stars contain a sea of free quark matter in their interiors. Our work could help tomorrow’s researchers determine whether such phase transitions actually occur.”
Gravitational waves can prove the existence of the quark-gluon plasma
Carolyn A. Raithel et al, Characterization of the breakdown of quasi-universality in gravitational waves after the fusion of binary neutron star mergers, The astrophysical diary letters (2022). DOI: 10.3847/2041-8213/ac7c75
Quote: New tool allows scientists to look inside neutron stars (2022, October 17) retrieved October 17, 2022 from https://phys.org/news/2022-10-tool-scientists-peer-neutron-stars.html
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