Imagine taking a star twice the mass of the Sun and crushing it to the size of Manhattan. The result would be a neutron star – one of the densest objects found anywhere in the universe, exceeding the density of any naturally occurring matter on Earth by a factor of tens of trillions. Neutron stars are unusual astrophysical objects in their own right, but their extreme density may also allow them to serve as laboratories for studying fundamental questions of nuclear physics, under conditions that could never be reproduced on Earth.
Because of these strange circumstances, scientists still do not understand exactly what neutron stars Themselves are made of the so-called “equation of state” (EoS). Determining this is the main objective of modern astrophysical research. A new piece of the puzzle, restricting the range of possibilities, has been discovered by two researchers at IAS: Caroline Raethel, John N. Bahkal Fellow in the College of Natural Sciences; And Elias Most, School Member and John A. Wheeler at Princeton University. Their work was recently published in Astrophysical Journal Letters.
Ideally, scientists would like to peek at these strange objects, but they are too small and too far to be imaged with standard telescopes. Scientists instead rely on indirect properties they can measure — such as a neutron star’s mass and radius — to calculate EoS, in the same way one might use the side length of a right-angled triangle to calculate the hypotenuse. However, it is very difficult to accurately measure the radius of a neutron star. One promising alternative for future observations is to use a quantity called “peak spectral frequency” (or f .).2) in its place.
But how is f2 measured? Collisions between neutron stars, governed by the laws of Einstein’s theory of relativity, lead to powerful outbursts of emission of gravitational waves. In 2017, scientists measured these emissions directly for the first time. “In principle at least, the spectral frequency peak can be calculated from the gravitational wave signal emitted by the oscillating remnants of two merging neutron stars,” Most says.
It was previously expected that f2 It would be a reasonable alternative to the radius, as researchers believe – so far – to be a direct or “near-universal” match between them. However, Raithel and Most prove that this is not always true. They show that identifying EoS is not the same as solving a simple chord problem. Instead, it’s more like calculating the longest side of an irregular triangle, as one also needs a third piece of information: the angle between the two shorter sides. For Raithel and Most, this third piece of information is the ‘mass-radius slope relationship’, which encodes information about EoS at higher densities (and therefore more extreme conditions) than the radius alone.
This new discovery will allow researchers to work with the next generation of gravitational-wave observatories (successors to the currently operating LIGO) to make better use of data obtained after merging neutron stars. According to Raithel, this data could reveal the basic components of neutron star matter. “some theoretical predictions Raythel points out that within the cores of neutron stars, phase transitions can dissolve neutrons into subatomic particles called quarks. This means that stars contain a sea of free quark matter within them. Our work may help tomorrow’s researchers determine if this is true Phase transitions already spoken.”
Carolyn A. Raethel et al., describe the quasi-universal collapse in Postmerger gravitational waves from Binary Neutron Star Mergers, Astrophysical Journal Letters (2022). DOI: 10.3847 / 2041-8213 / ac7c75
Institute of Advanced Studies
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