Three neutron stars reveal inner secrets: sky and telescopes

Astronomers using the XMM-Newton and Chandra space telescopes have revisited a trio of young neutron stars that are particularly cool for their age. Explaining their existence requires discarding 75% of all neutron star models, which will bring astronomers closer to identifying the correct one.

A neutron star is among the most exotic objects in the universe, forged by the death fury of a massive star. The star's core collapses under its own weight, crashing together with such force that electrons and protons are forced to fuse into neutrons. The resulting neutron star material is so dense that a single tablespoon of it would weigh more than all the human beings on Earth combined.

However, astronomers still don't know the exact structure of a neutron star, which likely includes electrons and protons in its crust and perhaps quarks in its core. The key to figuring out what's really inside neutron stars is identifying the right location. equation of state which describes the temperature and pressure inside all neutron stars. There are hundreds of possibilities.

Now, at last, astronomers have been able to narrow down that field. The team, led by Alessio Marino (Instituto de Ciencias del Espacio, Spain), studied a collection of 70 isolated neutron stars. Using X-ray measurements of the stars from XMM-Newton and Chandra, they estimated their temperatures. Crucially, some were still surrounded by supernova remnants, meaning the team was able to estimate their ages. All of those with age estimates were between 800 and 8,000 years old – they were astronomical infants. The team's findings are published in nature astronomy.

“Three of these neutron stars are much cooler than the others of similar ages,” says Marino. “This was a big clue that something strange might be happening inside these objects, something we need to understand.” The trio is particularly massive, meaning it contains more particles, and more particles mean more chances for unusual processes to occur, including some that could lead to rapid cooling.

The team used machine learning to sift through the multitude of possible equations of state to see which ones would allow for such rapid cooling. This process resulted in three-quarters of all models being ruled out. “If we are able to eliminate some of the possibilities about what is inside a neutron star, then the next question we need to ask is: what is left?” says team member Konstantinos Kovlakas (also Space Science Institute).

One possibility is that radioactive decay in the cores of neutron stars produces neutrinos, which help dissipate heat. Another possibility is that the stars are so massive that some of the central neutrons have decayed into their constituent quarks, either individually or bound together into particles known as mesons.

“We can’t say for sure what’s inside these neutron stars, but these latest data tell us that something exotic may be needed,” says team member Nanda Rea (also of the Space Science Institute).

“The early ages and low temperatures of these three have been known for 20 years, but this study is the first to systematically explore a large group of neutron star theories and outline which theories are ruled out,” says Craig Heinke (University of Alberta, Canada), who was not involved in the research.

One day we will have even better tools to help investigate these mysteries. “The (European Space Agency's) NewAthena mission will be much more sensitive than XMM-Newton, but it won't launch until around 2037,” says Heinke.

Chandra, on the other hand, is harder to replace. “Chandra's uniquely sharp vision means that several of the results of these discoveries could not have been obtained without it,” Heinke adds. With the recent and significant funding cut will likely lead to a premature end to the mission, he worries. “I hope funding can be found to sustain Chandra for another decade of exciting discoveries about neutron stars.”