Lessons from three neutron stars

ESA’s XMM-Newton probe and NASA’s Chandra probe have detected three young neutron stars that are unusually cool for their age. By comparing their properties to various models of neutron stars, the researchers concluded that the low temperatures of these oddballs disqualify about 75% of known models. This is a major step toward uncovering the one neutron star “equation of state” that rules them all, with important implications for the fundamental laws of the universe.

The work is published in a journal Astronomy of nature.

Mass squeezed to the extreme

After stellar black holes, neutron stars are the densest objects in the universe. Each neutron star is the compressed core of a giant star left over after the star exploded in a supernova. After running out of fuel, the star’s core implodes under the influence of gravity, while its outer layers are ejected into space.

The matter at the center of a neutron star is compressed so strongly that scientists still don’t know what form it takes. Neutron stars get their name from the fact that even atoms collapse under this immense pressure: electrons combine with atomic nuclei, turning protons into neutrons. But it could get even weirder, because the extreme heat and pressure can stabilize more exotic particles that can’t survive anywhere else, or possibly melt the particles together into a swirling soup of their quarks.

What happens inside a neutron star is described by a so-called “equation of state”, a theoretical model that describes what physical processes can occur inside a neutron star. The problem is that scientists don’t yet know which of the hundreds of possible equations of state models is correct. While the behavior of individual neutron stars can depend on properties such as their mass or how fast they spin, all neutron stars must follow the same equation of state.

Too cold

Examining data from ESA’s XMM-Newton and NASA’s Chandra missions, scientists discovered three exceptionally young and cold neutron stars that are 10-100 times cooler than their peers of the same age. By comparing their properties with the cooling rates predicted by various models, the researchers concluded that the existence of these three oddities rules out most of the proposed equations of state.

“The young age and cool surface temperature of these three neutron stars can only be explained by invoking a rapid cooling mechanism. Because enhanced cooling can only be activated by certain equations of state, this allows us to rule out a significant portion of possible models.” ” explains astrophysicist Nanda Rea, whose research group at the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC) led the investigation.

The discovery of the neutron star’s true equation of state also has important implications for the fundamental laws of the universe. Physicists don’t yet know how to connect general relativity (which describes the effects of gravity on large scales) with quantum mechanics (which describes what happens at the particle level). Neutron stars are the best testing ground for this because they have densities and gravities far beyond anything we can create on Earth.

Joining Forces: Four Steps to Discovery

Because the three strange neutron stars are so cold, they are too faint to be seen by most X-ray observatories. “The excellent sensitivity of XMM-Newton and Chandra made it possible not only to detect these neutron stars, but also to collect enough light to determine their temperatures and other properties,” says Camille Diez, an ESA researcher working on the XMM-Newton data.

However, the sensitive measurements were only the first step in drawing conclusions about what these oddities mean for the neutron star’s equation of state. To this end, Nanda’s research team at ICE-CSIC brought together the complementary expertise of Alessia Marino, Clara Dehman and Konstantinos Kovlakas.

Alessio led to the determination of the physical properties of neutron stars. The team could infer the neutron stars’ temperatures from the X-rays emitted from their surfaces, while the sizes and velocities of the surrounding supernova remnants provided precise data on their ages.

Additionally, Clara took the lead in calculating neutron star “cooling curves” for equations of state that include various cooling mechanisms. This includes plotting what each model predicts as the neutron star’s luminosity—a characteristic directly related to its temperature—changes over time.

The shape of these curves depends on several different properties of the neutron star, not all of which can be precisely determined from observations. For this reason, the team calculated cooling curves for a range of possible neutron star masses and magnetic field strengths.

In the end, it all came down to a statistical analysis led by Konstantinos. Using machine learning to determine how well the simulated cooling curves agree with the properties of the oddballs showed that equations of state without a rapid cooling mechanism have zero chance of matching the data.

“Neutron star research cuts across many scientific disciplines, from particle physics to gravitational waves. The success of this work shows how crucial teamwork is to advancing our understanding of the universe,” concludes Nanda.