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

ESA’s XMM-Newton and NASA’s Chandra spacecraft have detected three young neutron stars that are unusually cool for their age. By comparing their properties to those of different models of neutron stars, scientists conclude that the low temperatures of these stars disqualify about 75% of the known models. This is a big step toward discovering the neutron star “equation of state” that governs them all, with important implications for the fundamental laws of the universe.

The article is published in the journal Natural astronomy.

The material pressed to the extreme

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 as a supernova. After running out of fuel, the star’s core implodes under the force of gravity while its outer layers are flung into space.

The matter at the center of a neutron star is so tightly compressed 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, transforming protons into neutrons. But it could get even stranger, as the extreme heat and pressure could stabilize more exotic particles that don’t survive anywhere else, or possibly fuse the particles together in a swirling soup of their constituent quarks.

What happens inside a neutron star is described by what is known as the “equation of state”, a theoretical model that describes the physical processes that can occur inside it. a neutron star. The problem is that scientists don’t yet know which of the hundreds of possible state equation models is correct. Although the behavior of individual neutron stars can depend on properties such as their mass or the speed at which they rotate, all neutron stars must obey the same equation of state.

Too cold

Digging into data from ESA’s XMM-Newton and NASA’s Chandra missions, scientists discovered three exceptionally young and cool neutron stars, 10 to 100 times cooler than their peers of the same age. By comparing their properties to cooling rates predicted by different models, the researchers conclude that the existence of these three oddities 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. Since enhanced cooling can only be enabled 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 of Space Studies of Catalonia (IEEC) led the investigation.

Discovering the true equation of state of a neutron star also has important implications for the fundamental laws of the universe. It is common knowledge that physicists do not yet know how to combine the theory of general relativity (which describes the effects of gravity on a large scale) with quantum mechanics (which describes what happens at the particle level). Neutron stars provide the best testing ground for this, because they have densities and gravitation far beyond anything we can create on Earth.

Joining forces: four steps towards discovery

Because the three strange neutron stars are so cold, they are too dim for most X-ray observatories to see. “The superb 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,” explains Camille Diez, an ESA researcher who works on XMM-Newton data.

However, the sensitive measurements were only the first step toward being able to draw conclusions about what these oddities mean for the equation of state of neutron stars. 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 was able to infer the temperatures of neutron stars from X-rays emitted from their surfaces, while the size and speed of surrounding supernova remnants gave an accurate indication of their ages.

Next, Clara took the lead in calculating neutron star “cooling curves” for equations of state that incorporate different cooling mechanisms. This involves plotting what each model predicts about how a neutron star’s brightness – a characteristic directly linked 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 determined precisely 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 put it all together. Using machine learning to determine how well the simulated cooling curves align with the properties of the quirks showed that equations of state without rapid cooling mechanisms have no chance of matching the data.

“Research on neutron stars touches many scientific disciplines, from particle physics to gravitational waves. The success of this work demonstrates how teamwork is fundamental to advancing our understanding of the universe” , concludes Nanda.