Too Young to Be This Cold: Lessons from Three Neutron Stars

Compressed matter to the limit

After stellar-mass black holes, neutron stars are the densest objects in the Universe. Each neutron star arises from the core of a dead giant star that collapses after a supernova explosion, where after running out of fuel, the star’s core explodes under gravity and its outer layers are ejected into space.

At the center of a neutron star, matter is so compressed that scientists still don’t know what shape it takes. Neutron stars get their name because even atoms are destroyed under this enormous pressure: electrons merge with atomic nuclei, turning protons into neutrons. But it could be even weirder, since extreme heat and pressure can stabilize more exotic particles that don’t survive anywhere else, or even melt particles into a soup of their constituent quarks.

What happens inside a neutron star is explained by what is called an “equation of state,” a theoretical model that describes what physical processes can occur inside them. The problem is that scientists still don’t know which of the hundreds of possible state model equations is correct. Although the behavior of each neutron star may depend on properties such as its mass or rotation rate, all neutron stars must obey the same equation of state.

Too cold

Analyzing data from the European Space Agency’s XMM-Newton mission and NASA’s Chandra mission, scientists have discovered three exceptionally young and cool neutron stars: 10 to 100 times cooler than their similarly aged counterparts. By comparing their properties with the cooling rates estimated by various models, the researchers concluded that the existence of these three peculiar stars 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 a rapid cooling mechanism. Since this greater cooling can only be induced by certain equations of state, this allows us to rule out an important part of the possible models,” explains astrophysicist Nanda Rhee, whose research group at the Institute of Space Sciences (ICE-CSIC) and the Institute of Research was led by d’Estudis Espacials de Catalunya (IEEC).

The discovery of the true equation of state for neutron stars also has important implications for the fundamental laws of the Universe. Physicists still don’t know how to combine relativity (which describes the effects of gravity on a large scale) with quantum mechanics (which describes what happens at the particle level). Neutron stars are the best testing ground for this, since their density and gravitational forces far exceed anything we can create on Earth.

Joining forces: four steps to discovery

The fact that these three neutron stars are so cool makes them too faint to be detected by most X-ray observatories. “The extreme 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,” explains Camille Diez, an ESA researcher working on the XMM-Newton data. . Newton.

However, sensitive measurements were only the first step in deducing what these peculiar stars mean for the neutron star equation of state. To achieve this goal, Nanda’s research group at ICE-CSIC brought together the complementary expertise of Alessio Marino, Clara Deman and Konstantinos Kovlakas.

Alessio led the determination of the physical properties of neutron stars. The team was able to determine their temperature from the X-rays emitted from their surface, and the sizes and velocities of the surrounding supernova remnants gave an accurate idea of ​​their age.

Clara then calculated neutron star “cooling curves” for equations of state that included different cooling mechanisms. This involves plotting what each model predicts how the neutron star’s luminosity changes over time, a feature directly related to its temperature. The shape of these curves depends on various properties, 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, statistical analysis by Konstantinos put it all together. Using machine learning to determine the correspondence between modeled cooling curves and the properties of these rare stars showed that equations of state without a rapid cooling mechanism had no chance of matching the data.

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

Notes for reviewers

“Constraints on the equation of state for dense matter in young and cool isolated neutron stars” by Alessio Marino, Clara Deman, Konstantinos Kovlakas, Nanda Rhee, José A. Pons and Daniele Vigano is published in Nature Astronomy, DOI: 10.1038/s41550-024. -02291-у

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