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New tool lets scientists look inside neutron stars

New tool lets scientists look inside neutron stars

Neutron star merger and the gravitational waves it produces. Credit: NASA/Goddard Space Flight Center

Imagine taking a star twice the mass of the sun and crushing it to the size of Manhattan. The result will be a neutron star, one of the densest objects found anywhere in the universe, exceeding the density of any material naturally found on Earth by tens of trillions of times. Neutron stars are remarkable astrophysical objects in their own right, but their extreme density may also allow 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 are made of, the so-called “equation of state” (EoS). Determining this is the main goal of modern astrophysical research. A new piece of the puzzle that limits several possibilities has been discovered by a pair of scientists at IAS: Carolyn Raithel, Fellow of the John N. Bahcall School of Natural Sciences; and Elias Most, School Fellow and John A. Wheeler Fellow at Princeton University. Their work was recently published Astrophysical Journal Letters.

Ideally, scientists would like to peer inside these exotic objects, but they are too small and distant to be viewed with standard telescopes. Scientists rely on indirect properties they can measure (such as the mass and radius of a neutron star) to calculate EoS; you can likewise use the lengths of two sides of a right-angled triangle to calculate its hypotenuse. However, it is very difficult to measure the radius of a neutron star precisely. A promising alternative for future observations is to use a quantity called the “peak spectral frequency” (or f) instead.2) here.






In this animation, cursed neutron stars spin towards their death. Gravitational waves (pale arcs) leak orbital energy, causing stars to converge and merge. As the stars collide, some of the debris explodes in jets of particles traveling at nearly the speed of light, producing a short burst of gamma rays (magenta). In addition to the ultra-fast jets that power the gamma rays, the merger also produces slower-moving debris. An outlet driven by agglomeration on the fusion residue emits ultraviolet light (violet) that fades rapidly. A dense cloud of hot debris ejected from neutron stars just before the collision produces visible and infrared light (blue-white to red). UV, optical, and near-infrared radiation are collectively referred to as kilonovas. Later, X-rays (blue) were detected when the remnants of the jet headed towards us came into view. This animation represents phenomena observed up to nine days after GW170817. Credit: NASA’s Goddard Space Flight Center/CI Laboratory

but how f2 measured? Collisions between neutron stars, governed by the laws of Einstein’s Theory of Relativity, cause powerful bursts of gravitational wave emission. In 2017, scientists measured such emissions directly for the first time. “In principle, at least, the peak spectral frequency can be calculated from the gravitational wave signal emitted by the swinging remnant of two merged neutron stars,” Most says.

previously expected f2 would be a reasonable proxy for the radius, because – until now – researchers believed there was a direct or “semi-universal” correspondence between them. But Raithel and Most have shown that this is not always true. They showed that determining EoS is not like solving a simple hypotenuse problem. Instead, it’s more like calculating the longest side of an irregular triangle, for which a third piece of information is needed: the angle between the two shorter sides. For Raithel and Most, this third piece of information is the “slope of the mass-radius relationship”, which encodes information about EoS only at densities higher than the radius (and thus at more extreme conditions).

This new finding will allow researchers working with next-generation gravitational wave observatories (successors to the now-operating LIGO) to better use data from neutron star mergers. According to Raithel, these data could reveal key components of neutron star matter. “Some theoretical predictions suggest that within neutron star nuclei, phase transitions may resolve neutrons into subatomic particles called quarks,” Raithel said. “This means that stars contain a sea of ​​free quark matter within them. Our work could help tomorrow’s researchers determine whether such phase transitions actually occur.”


Gravitational waves could prove the existence of quark-gluon plasma


More information:
Carolyn A. Raithel et al, Distribution of Semi-universality in Post-merger Gravitational Waves from Binary Neutron Star Mergers, Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac7c75

Provided by the Institute for Advanced Study

Quotation: New instrument allows scientists to study the interior of neutron stars (October 17, 2022).

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