New tool uses gravitational waves to peer inside neutron stars

Imagine taking a star twice the mass of the Sun and crushing it down to the size of Manhattan. The result would be a neutron star – one of the densest objects in the universe. In fact, they exceed the density of all naturally occurring materials on Earth by a factor of tens of trillions. Although neutron stars are remarkable astrophysical objects in their own right, their extreme density could also allow them to act as laboratories to study fundamental questions in 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 astrophysical research. A new piece of the puzzle that limits the range of possibilities has been discovered by two researchers at the Institute for Advanced Study (IAS): Carolyn Raithel, John N. Bahcall Fellow in the School of Natural Sciences; and Elias Most, a member of the school and a John A. Wheeler Fellow

” data-gt-translate-attributes=”[{” attribute=””>Princeton University. Their paper was published recently in The Astrophysical Journal Letters.

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

Ideally, astrophysicists would like to look inside these exotic objects, but they are too small and distant to be imaged with standard telescopes. Researchers instead rely on indirect properties that they can measure—such as the mass and radius of a

Damned neutron stars whirl towards their demise in this animation. Gravitational waves (faint arcs) consume orbital energy, causing the stars to move closer together and merge. When the stars collide, some of the debris is ejected in jets of particles traveling at nearly the speed of light, producing a brief burst of gamma-rays (magenta). In addition to the ultra-fast jets that power the gamma rays, fusion also creates slower debris. Accretion-driven outflow onto the fusion remnant emits fast-fading ultraviolet (violet) light. A dense cloud of hot debris shed from the neutron stars just before the collision produces visible and infrared (blue-white to red) light. The glow in the UV, optical, and near-infrared regions is collectively referred to as a kilonova. Later, when the remnants of the jet aimed at us had extended into our line of sight, X-rays (blue) were detected. This animation represents phenomena observed up to nine days after GW170817. Recognition:

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It was previously expected that f₂ would be a reasonable proxy for radius, since—until now—researchers believed that a direct, or “quasi-universal,” correspondence existed between them. However, Raithel and Most have demonstrated that this is not always true. They have shown that determining the EoS is not like solving a simple hypotenuse problem. Instead, it is more akin to calculating the longest side of an irregular triangle, where 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 “slope of the mass-radius relation,” which encodes information about the EoS at higher densities (and thus more extreme conditions) than the radius alone.

This new finding will allow researchers working with the next generation of gravitational wave observatories (the successors of the currently operating LIGO) to better utilize the data obtained following neutron star mergers. According to Raithel, this data could reveal the fundamental constituents of neutron star matter. “Some theoretical predictions suggest that within neutron star cores, phase transitions could be dissolving the neutrons into sub-atomic particles called quarks,” stated Raithel. “This would mean that the stars contain a sea of free quark matter in their interiors. Our work may help tomorrow’s researchers determine whether such phase transitions actually occur.”

Reference: “Characterizing the Breakdown of Quasi-universality in Postmerger Gravitational Waves from Binary Neutron Star Mergers” by Carolyn A. Raithel and Elias R. Most, 13 July 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac7c75