Astronomers may have detected the "fingerprints" of axions, "ghost" particles that have been theorized for decades. As a reminder, these particles are also considered as candidates for the mysterious dark matter.
A neutron star forms when the core of a massive star collapses under the effect of gravity at the end of its life. Then emerges a small object a few kilometers in diameter whose density is of the order of a billion tons, composed almost entirely of neutrons tight against each other.
That being said, a cluster of neutron stars dubbed "The Magnificent Seven" is the subject of some questioning today. According to astronomers, it should indeed produce ultraviolet light and low-energy X-rays. However, a few years ago it was discovered that they also emit high-energy X-rays , which cannot be explained by our current models.
As part of a new study, a team of astronomers offers an intriguing explanation for this phenomenon.According to them, these signals could be generated by… axions.
Hypothetical particle axions were first proposed in 1977 to solve a cosmological conundrum called the strong CP problem. This one, by simplifying as much as possible, aims to understand why neutrons do not interact with electric fields.
A few years ago, axions were also proposed as promising candidates for dark matter, a form of matter thought to make up about 26.8% of the Universe that is not does not emit, absorb or reflect any light.
How then do we detect these "ghost" particles? For some researchers, the axions could interact with electromagnetic fields, such as those surrounding neutron stars. On paper, these particles could be created in large numbers at the core of these stellar corpses before making their way outwards, where the magnetic field would convert them into photons. Since axions carry large amounts of energy, so would their photons, potentially producing the high-energy X-rays seen around these stars.
At least, that's what this new study suggests. "Let's be clear, we don't claim to have discovered the axion yet. We simply evoke the idea that the additional X photons observed around this group of stars could be explained by the presence of axions” , underlines Raymond Co, of the University of Minnesota and lead author of the study. "This is an exciting possibility and consistent with our interpretation of these particles" .
For now, researchers are still wondering. And even if the observed X-ray excesses are not inherent in the presence of axions, the team points out that this work may still raise questions beyond the scope of the Standard Model, hinting at brand new physics.
Now it's time to continue researching. With that in mind, the team plans to leverage NASA's NuSTAR telescope to study this group of stars over a wider range of wavelengths. Magnetized white dwarf stars could also be targeted to look for axion emission. These objects indeed develop strong magnetic fields, and should not produce high-energy X-ray emissions.
Details of this work are published in the Physical Review Letters.
