Neutron stars are some of the densest objects in the universe. The material inside is squeezed so hard that scientists still don’t know what shape it takes. A neutron star’s core may be made of a thick soup of quarks or contain exotic particles that cannot survive anywhere else in the universe. Credit: ICE-CSIC/D. Futselaar/Marino et al., edited
Recent observations from ESA’s XMM-Newton and NASAChandra revealed three unusually cool, young neutron stars, challenging current models by showing that they are cooling much faster than expected.
This finding has significant implications, suggesting just 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 and NASA’s Chandra spacecraft have discovered three young neutron stars that are unusually cool for their age. By comparing their properties to various models of neutron stars, the scientists concluded that the low temperatures of the freaks disqualify about 75% of the known models. This is a major step toward uncovering the single neutron star “equation of state” that governs them all, with important implications for the fundamental laws of the universe.
Aside from black holes, neutron stars are among the most perplexing objects in the universe. A neutron star forms in the final moments of the life of a very large star (more than about 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. At the center of the cloud (nebula), the dense stellar core further contracts 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-mass black holes, neutron stars are the most dense objects in the universe. Each neutron star is the compressed core of a giant star left over from the star’s supernova explosion. After running out of fuel, the star’s core collapses under the force of gravity, while its outer layers are shot out into space.
The matter at the center of a neutron star is squeezed so hard that scientists still don’t know what shape it takes. Neutron stars get their name from the fact that under this enormous pressure, even atoms collapse: electrons fuse with atomic nuclei, turning protons into neutrons. But it can get even stranger, as the extreme heat and pressure can stabilize more exotic particles that don’t survive anywhere else, or possibly melt the particles together into a swirling soup of their constituent 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 still don’t know which of the hundreds of possible equations of state models is correct. While the behavior of individual neutron stars may depend on properties such as their mass or how fast they spin, all neutron stars must obey the same equation of state.
Implications of observations of neutron star cooling
Digging into data from ESA’s XMM-Newton and NASA’s Chandra missions, scientists discovered three extremely young and cold neutron stars that are 10–100 times colder 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 oddballs rules out most of the proposed equations of state.
“The young age and cold surface temperature of these three neutron stars can only be explained by invoking a rapid cooling mechanism. Because the enhanced cooling can only be activated by certain equations of state, this allows us to rule out a significant number of possible models,” explains astrophysicist Nanda Rhea, whose research group at the Institute for Space Sciences (ICE-CSIC) and the Institute for Space Studies of Catalonia (IEEC) are leading the investigation.
Unifying theories through the study of neutron stars
Unraveling the true equation of state of the neutron star also has important implications for the fundamental laws of the universe. It is known that physicists still do not know how to connect the theory of general relativity (which describes the effects of gravity on large scales) with quantum mechanics (which describes what happens at the level of particles). Neutron stars are the best testing ground for this, as they have densities and gravities far beyond anything we can create on Earth.
Neutron stars are the compressed cores of giant stars left over after the star explodes in a supernova. They are so dense that the amount of material in a neutron star like a sugar cube would weigh as much as all the people on Earth! Credit: ESA
Joining Forces: Four Steps to Discovery
The three strange neutron stars are so cold that they are too dark to be seen by most X-ray observatories. “The superior 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,” said Camille Diez, ESA research scientist who worked on the data by XMM-Newton.
However, the sensitive measurements were only the first step towards being able to draw conclusions about what these oddballs mean for the neutron star’s equation of state. To this end, Nanda’s research team at ICE-CSIC combined the complementary expertise of Alessio Marino, Clara Dehman and Konstantinos Kovlakas.
Alessio led the determination of the physical properties of neutron stars. The team could infer the temperatures of the neutron stars from the X-rays emitted from their surfaces, while the sizes and velocities of the surrounding supernova remnants gave an accurate indication of their age.
Clara then took the lead in calculating neutron star “cooling curves” for equations of state that include various cooling mechanisms. This involves plotting what each model predicts about how the neutron star’s brightness – a characteristic directly related to its temperature – changes over time. The shape of these curves depends on several different properties of a 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.
Finally, a statistical analysis led by Konstantinos brought it all together. Using machine learning to determine how well the simulated cooling curves align with the oddball properties showed that equations of state without a rapid cooling mechanism have zero chance of matching the data.
“Neutron star research crosses many scientific disciplines, ranging from particle physics to gravitational waves. The success of this work demonstrates how fundamental teamwork is to advancing our understanding of the universe,” concludes Nanda.
Reference: “Constraints on the equation of state of dense matter from young and cold isolated neutron stars” by A. Marino, C. Dehman, K. Kovlakas, N. Rea, JA Pons and D. Viganò, 20 Jun 2024, Natural astronomy.
DOI: 10.1038/s41550-024-02291-y
