A new tool can shed new light on the interior of merging neutron stars

Neutron stars are among the densest objects in the known universe as they typically condense masses several times that of our Sun into a size smaller than the Earth.

Though we know a good amount about their characteristics, such as the fact they are the collapsed cores of massive supergiant stars, scientists are yet to pin down precisely what the exotic space objects are made of.

A pair of scientists believes it has narrowed down the possibilities for the so-called "equation of state" (EoS) of neutron stars, which determines the composition of a neutron star, a press statement reveals.

Investigating neutron stars

Neutron stars are typically so distant and small that scientists rely on indirect properties — such as their mass and radius — to measure their EoS. The pair of scientists, whose work is outlined in a paper in The Astrophysical Journal Letters, proposed using a quantity called the "peak spectral frequency" (or f2) as an alternative for measuring the radius of small, distant space objects.

The f2 of a neutron star is measured via strong bursts of gravitational wave emissions emitted when two neutron stars collide. Scientists first measured such emissions in 2017.

"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," explained Elias Most, one of the scientists behind the new paper.

It was previously thought that f2 could serve as a good proxy for radius since researchers believed a direct correspondence existed between the two. Now, the new paper's two authors have revealed that this is not always the case. The press stated, "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."

According to the study authors, this third piece of information is the "slope of the mass-radius relation," which encodes information about the EoS at higher densities than the radius.

The next generation of gravitational wave research

The new study could help guide future research conducted on neutron star mergers using the next-generation gravitational wave observatories that will succeed NASA's Laser Interferometer Gravitational Wave Observatory (LIGO). LIGO has also been used to detect massive black hole mergers that similarly send gravitational waves cascading through the universe.

Ultimately, the two scientists behind the new paper believe their new method could be used to peer inside a neutron star to ascertain its composition essentially. As co-author Carolyn Raithel points out, "some theoretical predictions suggest that within neutron star cores, phase transitions could be dissolving the neutrons into sub-atomic particles called quarks. 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."