New Tool Uses Gravitational Waves to Peer Inside Neutron Stars

Imagine taking a star with twice the mass of the Sun and crushing it all the way down to the dimensions of Manhattan. The consequence can be a neutron star—one of many densest objects discovered anyplace within the Universe. In reality, they exceed the density of any materials discovered naturally on Earth by an element of tens of trillions. Although neutron stars are outstanding astrophysical objects in their very own proper, their excessive densities can also enable them to perform as laboratories for finding out basic questions of nuclear physics, below situations that might by no means be reproduced on Earth.

Because of those unique situations, scientists nonetheless don’t perceive what precisely neutron stars themselves are comprised of, their so-called “equation of state” (EoS). Determining this can be a main aim of recent astrophysics analysis. A brand new piece of the puzzle, constraining the vary of prospects, has been found by a pair of students on the Institute for Advanced Study (IAS): Carolyn Raithel, John N. Bahcall Fellow within the School of Natural Sciences; and Elias Most, Member within the School and John A. Wheeler Fellow at 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 neutron star—to calculate the EoS. This is much like how one might use the length of two sides of a right-angled triangle to work out its hypotenuse. However, one issue here is that the radius of a neutron star is very difficult to measure precisely. A promising alternative for future observations is to instead use a quantity called the “peak spectral frequency” (or f₂) in its place.

But how is f₂ measured? Collisions between neutron stars, which are governed by the laws of Einstein’s Theory of Relativity, lead to strong bursts of gravitational wave emission. In 2017, scientists directly measured such emissions for the first time. “At least in principle, the peak spectral frequency can be calculated from the gravitational wave signal emitted by the wobbling remnant of two merged neutron stars,” says Most.

Doomed neutron stars whirl towards their demise on this animation. Gravitational waves (pale arcs) bleed away orbital vitality, inflicting the celebrities to maneuver nearer collectively and merge. As the celebrities collide, a few of the particles blasts away in particle jets transferring at almost the pace of sunshine, producing a short burst of gamma rays (magenta). In addition to the ultra-fast jets powering the gamma rays, the merger additionally generates slower-moving particles. An outflow pushed by accretion onto the merger remnant emits quickly fading ultraviolet mild (violet). A dense cloud of sizzling particles stripped from the neutron stars simply earlier than the collision produces seen and infrared mild (blue-white by means of pink). The UV, optical, and near-infrared glow is collectively known as a kilonova. Later, as soon as the remnants of the jet directed towards us had expanded into our line of sight, X-rays (blue) had been detected. This animation represents phenomena noticed as much as 9 days after GW170817. Credit: NASA’s Goddard Space Flight Center/CI Lab

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