Astronomers surveyed dozens of neutron stars, homing in on three that challenge most ideas about what these exotic objects are made of.

Artist's illustration of a magnetar
Artist’s illustration of a neutron star.

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 ruling out 75% of all neutron star models — bringing astronomers closer to identifying the correct one.

A neutron star is among the universe’s most exotic objects, forged in the fury of a massive star’s death. The star’s core buckles under its own weight, crashing down so hard that electrons and protons are forced to merge into neutrons. The resulting neutron star material is so dense that a single spoonful would weigh more than every human on Earth put together.

Yet astronomers still don’t know the exact structure of a neutron star, which probably includes electrons and protons in its crust and maybe quarks in its core. The key to finding out what’s really inside neutron stars is identifying the correct equation of state that describes the temperature and pressure of all neutron star interiors. There are hundreds of possibilities.

Now, finally, astronomers have been able to narrow that field. The team, led by Alessio Marino (Institute of Space Sciences, Spain), studied a collection of 70 isolated neutron stars. Using XMM-Newton’s and Chandra’s measurements of the stars’ X-rays, they estimated their temperatures. Crucially, some were still surrounded by supernova remnants, meaning the team could estimate their ages. All of those with age estimates were between 800 and 8,000 years old — astronomical infants. The team's findings are published in Nature Astronomy.

3C 58 supernova remnant
This image of the supernova remnant 3C 58 contains a white-hot neutron star at its center (white). That neutron star, however, is cooler than expected compared to its siblings elsewhere in the galaxy. Around it in the enter is a torus filled with speedy charged particles and a jet. The image is colorized by the energy of the X-rays (low-energy X-rays are red, medium-energy are green, and high-energy are blue).
X-ray: NASA / CXC / ICE-CSIC / A. Marino et al.; Optical: SDSS; Image Processing: NASA / CXC / SAO / J. Major

“Three of these neutron stars are much cooler than the others at similar ages,” Marino says. “This was a big clue that something weird might be going on inside these objects, which we need to understand.” The trio are particularly massive, meaning they contain more particles, and more particles means more chances for unusual processes to occur — including ones that could lead to rapid cooling.

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

One possibility is that radioactive decay in neutron star cores produces neutrinos, which help carry away heat. Alternatively, the stars may be so massive that some of the central neutrons have broken down into their constituent quarks, either individually or bound together in particles known as mesons.

“We cannot say with certainty what is inside of these neutron stars, but these latest data are telling us that something exotic may be needed,” says team member Nanda Rea (also Institute of Space Sciences).

Neutron star interior
Scientists think neutron stars are layered, with the above diagram suggesting one, simplified view of those layers' compositions. The state of matter in their inner cores remains unknown.
NASA’s Goddard Space Flight Center / Conceptual Image Lab

“The young ages and cold 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 delineate which theories are ruled out,” says Craig Heinke (University of Alberta, Canada), who was not involved in the research.

One day, we’ll have even better tools to help probe these mysteries. “The [European Space Agency’s] NewAthena mission will be much more sensitive than XMM-Newton, but won’t be launched until about 2037,” Heinke says.

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


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