Astronomers studying types of spinning neutron stars known as pulsars have discovered that their interiors may contain a kind of ultra-dense matter not found anywhere else in the Universe.
The study looked at the remnants of a stellar explosion, known as supernova remnant 3C 58, and the pulsar lurking within it.
Astronomers examined pulsar PSR J0205+6449 at the supernova remnant's centre, and other pulsars like it, using the Chandra X-ray Observatory and ESA’s XMM-Newton telescope.
They analysed previously released data on neutron stars to determine something called 'equation of state'.
This refers to certain properties of neutron stars such as pressure and temperature in different parts of their interiors.
And they used A.I. machine learning to compare the data of different equations of state.
Some of the neutron stars studied, including 3C 58, are located in supernova remnants, and because astronomers have estimates of the ages of the remnants, they have age estimates for the neutron stars that were created by the supernova explosions.
Cooler than expected
They found the neutron star in 3C 58, and two neutron stars, much cooler than the rest of the neutron stars in the study.
This could be because those neutron stars are more massive than most of the rest, which means they have more particles, and certain processes have been triggered that cause the neutron stars to cool more rapidly than others.
Or it could mean a sort of radioactive decay is occurring near the centres of these rapidly-cooling neutron stars.
Or perhaps there is some kind of mysterious, unknown high-dense matter lurking within them.
Science takeaways
"Three of these neutron stars are much cooler than the others at similar ages," says Alessio Marino, of the Institute of Space Sciences (ICE) in Barcelona, Spain, author of a paper published in Nature Astronomy describing the results.
"This was a big clue that something weird might be going on inside these objects, which we need to understand."
"Because more massive neutron stars have more particles, special processes that cause neutron stars to cool more rapidly might be triggered," says co-author Clara Dehman, also of ICE.
"It’s like having early answers filled in on a crossword puzzle – it makes filling in the rest of the answers much easier."
"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 co-author Nanda Rea of ICE.
"Understanding the structure and properties of neutron stars might be crucial for other fields of astrophysics, such as interpreting the bursts of gravitational waves when they merge."
The cool three neutron stars are PSR J0205+6449, PSR B2334+61, and CXOU J0852−4617 (PSR J0205, PSR B2334, and CXOU J0852 for short).
PSR J0205 is in the supernova remnant 3C 58, PSR B2335 in SNR G114.3+0.3, and CXOU J0852 in SNR Vela Jr.
Read the full paper at www.nature.com/articles/s41550-024-02291-y
