Neutron stars are some of the densest objects in the universe. The material inside is compressed so strongly that scientists do not yet know what form it takes. A neutron star’s core may be made of a thick soup of quarks, or it may contain exotic particles that could not survive anywhere else in the universe. Credit: ICE-CSIC/D. Futselaar/Marino et al., ed
Recent observations of the ESA XMM-Newton and NASA‘s Chandra revealed three unusually cool young neutron stars that challenge current models by showing that they are cooling much faster than expected.
This finding has significant implications, suggesting that only a few of the many proposed neutron star models are viable and point to a potential breakthrough in linking the theories of general relativity and quantum mechanics through astrophysical observations.
Discovery of unusually cold 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.
Apart from black holes, neutron stars are among the most mysterious objects in the universe. A neutron star is formed in the final moments of the life of a very large star (more than eight times the mass of our Sun), when the nuclear fuel in its core eventually runs out. In a sudden and violent end, the star’s outer layers are ejected with monstrous energy in a supernova explosion, leaving behind spectacular clouds of interstellar material rich in dust and heavy metals. In the center of the cloud (nebula), the dense stellar core continues to contract to form a neutron star. A black hole can also form when the mass of the remaining core is greater than about three solar masses. Credit: ESA
Extreme density and unknown states of matter
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.
In a neutron star (left), the quarks that make up the neutrons are trapped inside the neutrons. In a quark star (right), the quarks are free, so they take up less space and the diameter of the star is smaller. Credit: NASA/CXC/M.Weiss
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.
Implications of observations of neutron star cooling
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 low age and low surface temperature of these three neutron stars can only be explained by the use of a rapid cooling mechanism. Since enhanced cooling can only be activated by certain equations of state, it allows us to rule out a significant part of possible models,” explains astrophysicist Nanda Rea, whose research group at the Institute of Space Sciences (ICE-CSIC) and the Institute for Space Research led the studies of Catalonia ( IEC).
Unifying theories through the study of neutron stars
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.
Neutron stars are the compressed cores of giant stars left over from the explosion of a star in a supernova. They are so dense that the amount of neutron star material in a sugar cube would weigh as much as all the people on Earth! Credit: ESA
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. By 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 fast 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.
Reference: “Constraints on the dense matter equation of state from young and cold isolated neutron stars” by A. Marino, C. Dehman, K. Kovlakas, N. Rea, JA Pons and D. Viganò, 20 Jun 2024, Astronomy of nature.
DOI: 10.1038/s41550-024-02291-y