Now we can see inside Neutron stars. What about black holes?

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Now we can see inside Neutron stars. What about black holes?

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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 found in the universe, exceeding the density of any matter found naturally on Earth by tens of trillions. Neutron stars are extraordinary astrophysical objects in their own right, but their extreme density may also allow them to function as a laboratory for studying fundamental questions of physics. nuclear, under conditions that could never be reproduced on Earth.

Because of these strange conditions, scientists still don’t understand exactly what neutron stars themselves, their so-called “equation of state” (EoS), are made of. Identifying this is a major goal of modern astrophysics research. A new piece of the puzzle, limiting the range of possibilities, has been explored by two IAS researchers: Carolyn Raithel, John N. Bahcall Fellow at the School of Natural Sciences; and Elias Most, School Fellow and John A. Wheeler Scholar at Princeton University. Their work was recently published in the Astrophysical Journal Letters.

Ideally, scientists would like to look inside these strange objects, but they are too small and far away to be imaged with standard telescopes. Instead, scientists rely on indirect properties they can measure – like the mass and radius of a neutron star – to calculate EoS, just as one might use two-sided lengths. of a right triangle to find its hypotenuse. However, the radius of a neutron star is difficult to measure accurately. A promising alternative for future observations is to use a quantity known as “peak spectral frequency” (or f2) instead.

Deadly neutron stars spin towards their demise in this animation. Gravitational waves (pale arcs) melt orbital energy, causing the stars to come closer together and merge. When the stars collide, some of the debris explodes into jets of particles traveling close to the speed of light, creating a brief burst of gamma rays (magenta). In addition to the extremely fast jets delivering gamma rays, the merger also produces slower-moving debris. The flow in the direction of accretion on the remains of the melt emits ultraviolet (violet) light that rapidly fades. A dense cloud of hot debris ripped from neutron stars just before the collision produces visible and infrared (blue-white to red) light. UV, optical, and near-infrared rays are collectively known as kilonova. Then, as the remnants of the jet aimed at us expanded in our view, X-rays (in blue) were detected. This animation represents phenomena observed up to nine days after GW170817. Image supplier: NASA’s Goddard Space Flight Center / CI . Laboratories

But how is f2 measured? Collisions between neutron stars, governed by Einstein’s law of relativity, lead to explosive gravitational waves. In 2017, scientists directly measured these emissions for the first time. “At least in principle, the peak spectral frequency can be calculated from the gravitational wave signal emitted by the oscillating remnants of two merging neutron stars,” Most said.

It was previously predicted that f2 would be a reasonably approximate radius, because – so far – researchers believe there is a direct or “near-universal” correspondence between them. However, Raithel and Most have shown that this is not always the case. They showed that determining EoS is not the same as solving a simple hypotenuse problem. Instead, it’s like calculating the longer side of an irregular triangle, where you also need 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 relationship”, which encodes information about EoS at a higher density (and therefore more severe conditions) than radius.

This new discovery will allow researchers to work with the next generation of gravitational wave observatories (successors of the currently working LIGO) to make better use of the data obtained by stellar mergers. neutrons. According to Raithel, these data could reveal the basic composition of neutron star matter. “Some theoretical predictions suggest that in the cores of neutron stars, phase transitions can decompose neutrons into subatomic particles called quarks,” says Raithel. “This means that stars contain a sea of ​​free quark matter inside. Our work could help researchers tomorrow determine whether such phase transitions really do occur.

More information: Carolyn A. Raithel et al, Characterizing the Breakdown of Quasi-universality in Postmerger Gravitational Waves from Binary Neutron Star Mergers, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac7c75

Journal information: Astrophysical Journal Letters 

Provided by Institute for Advanced Study 

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