New tool allows scientists to look inside neutron stars

Imagine taking a star twice the size 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, exceeding the density of all naturally occurring material on Earth by a factor of tens of trillions. Neutron stars are extraordinary astrophysical objects in their own right, but their extreme density could also allow them to act as laboratories for studying 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 scientists at the 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 at Princeton University. Her work was recently published in The Letters of the Astrophysical Journal.

Scientists would love to peer into these exotic objects, but they are too small and too distant to be imaged with standard telescopes. Scientists instead rely on indirect properties they can measure — like a neutron star’s mass and radius — to calculate the EoS, just as one might use the length of two sides of a right triangle to calculate its hypotenuse. However, the radius of a neutron star is very difficult to measure accurately. A promising alternative for future observations is to instead use a quantity called “Peak Spectral Frequency” (or f₂) at his place.

But how is f₂ measured? Collisions between neutron stars, obeying the laws of Einstein’s theory of relativity, result in powerful bursts of gravitational-wave emission. In 2017, scientists measured such emissions directly 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 neutron star mergers,” says Most.

Previously it was expected that f₂ would be a reasonable substitute for Radius, since researchers until now believed there was a direct or “quasi-universal” correspondence between them. However, Raithel and Most have shown that this is not always the case. They have shown that the determination of the EoS is Not like solving a simple hypotenuse problem. Instead, it’s more like calculating the longest side from an irregular Triangle, where you 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 the EoS at higher densities (and thus more extreme conditions) than radius alone.

This new finding will allow researchers working with next-generation gravitational-wave observatories (successors to the currently operational LIGO) to make better use of the data obtained after neutron star mergers. According to Raithel, this data could reveal the basic constituents of neutron star matter. “Some theoretical predictions suggest that phase transitions within neutron star cores could break up the neutrons into subatomic particles called quarks,” Raithel said. “That would mean that the stars contain a sea of ​​free quark matter inside. Our work can help tomorrow’s researchers determine whether such phase transitions actually occur.”

About the institute

Since its inception in 1930, the Institute for Advanced Study has served the world as one of the leading independent centers for theoretical research and intellectual inquiry, pushing the frontiers of knowledge in the sciences and humanities. From the work of the founding faculty of the IAS, such as Albert Einstein and John von Neumann, to the leading thinkers of today, the IAS is dedicated to enabling curious inquiry and fundamental discovery.

Each year, the institute welcomes more than 200 of the world’s most promising postdocs and researchers, selected and mentored by a permanent faculty, each of whom is an outstanding leader in their field. Present and past faculty and members have included 35 Nobel Prize winners, 44 of the 62 Fields Medalists, and 22 of the 25 Abel Prize winners, as well as many MacArthur Fellows and Wolf Prize winners.

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