Neutron stars are among the densest objects in the Universe. The matter they are made of is so compressed that scientists do not yet know what shape it takes. The core of a neutron star could consist of a thick soup of quarks or contain exotic particles that could not survive anywhere else in the Universe. Credit: ICE-CSIC/D. Futselaar/Marino et al., edited
Recent observations made by XMM-Newton and
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>NASAChandra researchers have revealed three unusually cool and young neutron stars, challenging current models by showing that they are cooling much faster than expected.
This discovery has important implications, suggesting that only a few of the many proposals
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>neutron star The models are viable and indicate a potential breakthrough in linking the theories of general relativity and quantum mechanics through astrophysical observations.
Unusually cold neutron stars discovered
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 those of different neutron star models, scientists conclude that the low temperatures of these neutron stars rule out about 75% of the known models. This is a major step towards discovering the “equation of state” of neutron stars that governs them all, with important implications for the fundamental laws of the Universe.
Next to black holes, neutron stars are among the most puzzling objects in the Universe. A neutron star forms in the final moments of the life of a very large star (about eight times the mass of our Sun), when the nuclear fuel in its core 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 matter rich in dust and heavy metals. At the centre of the cloud (nebula), the dense stellar core contracts further 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 densest objects in the Universe. Each neutron star is the compressed core of a giant star, left behind after the star explodes in a supernova. Once it runs out of fuel, the star’s core implodes under gravity while its outer layers are flung out into space.
The matter at the center of a neutron star is compressed to such an extent that scientists still don’t know what shape it takes. Neutron stars get their name because under this immense pressure, even atoms collapse: electrons fuse with atomic nuclei, turning protons into neutrons. But it could get even weirder, because the extreme heat and pressure could stabilize more exotic particles that don’t survive anywhere else, or perhaps melt the particles together to form a swirling soup of quarks that make them up.
In a neutron star (left), the quarks that make up the neutrons are confined 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 something called the “equation of state,” a theoretical model that describes the physical processes that can occur inside a neutron star. The problem is that scientists don’t yet know which of the hundreds of possible equation of state models is correct. While the behavior of individual neutron stars can depend on properties like their mass or how fast they spin, all neutron stars must obey the same equation of state.
Consequences of observations on the cooling of neutron stars
Analyzing data from ESA’s XMM-Newton and NASA’s Chandra missions, scientists have discovered three exceptionally young and cool neutron stars, 10 to 100 times cooler than their counterparts of the same age. By comparing their properties to the cooling rates predicted by different models, the researchers conclude that the existence of these three eccentrics rules out most of the proposed equations of state.
“The young age and low surface temperature of these three neutron stars can only be explained by invoking a rapid cooling mechanism. Since accelerated cooling can only be activated by certain equations of state, this allows us to exclude a significant part of the possible models,” explains astrophysicist Nanda Rea, whose research group at the Institute of Space Sciences (ICE-CSIC) and the Institute for Space Studies of Catalonia (IEEC) led the study.
Unifying theories through the study of neutron stars
The discovery of the true equation of state of neutron stars also has important implications for the fundamental laws of the universe. Physicists don’t yet 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 particle level). Neutron stars are the best testing ground for this, because they have densities and gravitation 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
These three bizarre neutron stars are so cool that they are too faint to be observed 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 temperature and other properties,” says Camille Diez, an ESA researcher working on the XMM-Newton data.
These precise measurements are, however, only a first step towards drawing conclusions about the implications of these strange phenomena for the equation of state of neutron stars. To this end, Nanda’s research team at ICE-CSIC has brought together the complementary expertise of Alessio Marino, Clara Dehman and Konstantinos Kovlakas.
Alessio has been researching the physical properties of neutron stars. The team was able to deduce the temperature of neutron stars from the X-rays emitted from their surfaces, while the size and speed of surrounding supernova remnants gave a precise indication of their age.
Clara then took it upon herself to calculate neutron star “cooling curves” for equations of state that incorporate different cooling mechanisms. This involves plotting each model’s predictions of how a 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 a neutron star, not all of which can be accurately 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 conducted by Konstantinos put it all together.
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>machine learning To determine how well the simulated cooling curves align with the properties of the eccentrics, it was shown that equations of state without a rapid cooling mechanism have no chance of fitting the data.
“Neutron star research spans many scientific disciplines, from particle physics to
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>gravitational waves“The success of this work demonstrates how fundamental teamwork is to advance our understanding of the Universe,” Nanda concludes.
Reference: “Constraints on the equation of state of dense matter in isolated young and cold neutron stars” by A. Marino, C. Dehman, K. Kovlakas, N. Rea, JA Pons and D. Viganò, June 20, 2024, Astronomy of nature.
DOI: 10.1038/s41550-024-02291-y