Neutron stars are among the densest objects in the universe. The material inside them is so tightly compressed that scientists don’t yet know what shape it takes. The core of a neutron star may be 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., modified
Recent observations by ESA’s XMM-Newton and NASAChandra have discovered three unusually cold, young neutron stars, challenging current models by showing that they are cooling much faster than expected.
This finding has important implications and suggests that only a few of the many proposed neutron star models are viable and point to a potential breakthrough in connecting the theories of general relativity and quantum mechanics via 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 cold for their age. By comparing their properties with different neutron star models, scientists conclude that the low temperatures of the oddballs disqualify about 75% of known models. This is a major step toward revealing the one neutron star ‘equation of state’ that governs them all, with important implications for the fundamental laws of the Universe.
Next to black holes, neutron stars are among the most baffling objects in the universe. A neutron star is formed 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 finally runs out. In a sudden and violent end, the outer layers of the star 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 shrinks further to form a neutron star. A black hole can also form when the mass of the remaining core exceeds about three solar masses. Credit: ESA
Extreme density and unknown states of matter
After stellar-mass black holes, neutron stars are the densest objects in the universe. Each neutron star is the compressed core of a giant star, left behind after the star exploded in a supernova. After running out of fuel, the star’s core implodes under gravity, while its outer layers are blown out into space.
Matter at the center of a neutron star is compressed so much that scientists still don’t know what shape it takes. Neutron stars get their name from the fact that under this immense pressure, even atoms collapse: electrons fuse with atomic nuclei, turning protons into neutrons. But it can get even weirder, as the extreme heat and pressure can stabilize more exotic particles that can’t survive anywhere else, or possibly fuse particles together into a swirling soup of their constituent quarks.
In a neutron star (left), the quarks that make up the neutrons are locked 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 the so-called ‘equation of state’, a theoretical model that describes what physical processes can happen inside a neutron star. The problem is that scientists do not yet know which of the hundreds of possible equation of state models is correct. Although the behavior of individual neutron stars can 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
By digging into 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 colder than their peers. By comparing their properties with the cooling rates predicted by different models, the researchers conclude that the existence of these three oddballs rules out most 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. Since enhanced cooling can only be triggered by certain equations of state, this allows us to rule out a significant fraction of possible models,” explains astrophysicist Nanda Rea, whose research group at the Institute of Space Sciences (ICE-CSIC) and Institute of Space Studies of Catalonia (IEEC) led the study.
Unifying theories through neutron star research
Uncovering the true equation of state of neutron stars also has important implications for the fundamental laws of the universe. Physicists still don’t know how to connect general relativity (which describes the effects of gravity on large scales) to quantum mechanics (which describes what happens at the level of particles). Neutron stars are the best testing ground for this, because they have densities and gravity that are far beyond anything we can create on Earth.
Neutron stars are the compressed cores of giant stars, left behind after the star explodes 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
The three strange neutron stars are so cold that they are too faint for most X-ray observatories to see. “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,” says Camille Diez, an ESA researcher working on XMM-Newton data.
However, the sensitive measurements were only the first step in drawing conclusions about what these oddballs mean for the equation of state of neutron stars. To achieve this goal, 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 the 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 a precise indication of their ages.
Clara then took the lead in calculating neutron star “cooling curves” for equations of state that include different cooling mechanisms. This involves plotting what each model predicts about how a neutron star’s luminosity—a property directly related to its temperature—changes over time. The shape of these curves depends on several properties of a neutron star, not all of which can be determined accurately 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. machine learning To determine to what extent the simulated cooling curves match the properties of the odd ones out, it was shown that equations of state without a rapid cooling mechanism have no chance of matching the data.
“The study of neutron stars spans many scientific disciplines, from particle physics to gravitational waves“The success of this work shows how fundamental teamwork is to advancing our understanding of the universe,” Nanda concludes.
Reference: “Constraints on the dense matter equation of state of young and cold isolated neutron stars” by A. Marino, C. Dehman, K. Kovlakas, N. Rea, JA Pons and D. Viganò, June 20, 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02291-y