First theorized in the 1970s, axions are hypothetical particles that were proposed to preserve a time-reversal symmetry of the nuclear force. These particles may make up dark matter and may be produced thermally inside the cores of neutron stars, escape the stars due to their feeble interactions with matter, and subsequently convert into X-rays in the magnetic fields surrounding the stars. In a paper in the journal Physical Review Letters, astrophysicists show that a recently discovered excess of hard X-rays from a nearby group of isolated neutron stars, known as the Magnificent Seven, could be explained by this emission mechanism.
The Magnificent Seven is a group of isolated X-ray dim neutron stars at a distance of 400 to 2,160 light-years from Earth.
These stars possess powerful magnetic fields, and were only expected to produce low-energy X-rays and UV light.
“They are known to be very ‘boring,’ and in this case it’s a good thing,” said Dr. Benjamin Safdi, a theoretical physicist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.
If axions exist, they would be expected to behave much like neutrinos in a star, as both would have very slight masses and interact only very rarely and weakly with other matter. They could be produced in abundance in the interior of stars.
Uncharged particles called neutrons move around within neutron stars, occasionally interacting by scattering off of one another and releasing a neutrino or possibly an axion. The neutrino-emitting process is the dominant way that neutron stars cool over time.
Like neutrinos, the axions would be able to travel outside of the star. The incredibly strong magnetic field surrounding the Magnificent Seven stars could cause exiting axions to convert into light.
“The axion was first proposed in the late 1970s to solve a problem called the strong CP problem, which means the negative and positive electric charge distributions inside the neutron are centered around the same point,” said Christopher Dessert, a graduate student at the University of Michigan.
“In the next decade, it was discovered that if the axion existed, it could also be dark matter.”
In 2019, astrophysicists observed a mysterious, inexplicable increase in X-rays emitted from the Magnificent Seven.
Dr. Safdi, Dessert and their colleagues propose that these extra X-rays are caused by axions being produced in the neutron stars’ cores.
If the X-ray excess is generated from an object or objects hiding out behind the neutron stars, that likely would have shown up in the datasets that the researchers are using from two space satellites: ESA’s XMM-Newton and NASA’s Chandra X-ray telescopes.
“It’s still quite possible that a new, non-axion explanation arises to account for the observed X-ray excess, though we remain hopeful that such an explanation will lie outside of the Standard Model of particle physics, and that new ground- and space-based experiments will confirm the origin of the high-energy X-ray signal,” Dr. Safdi said.
“We are pretty confident this excess exists, and very confident there’s something new among this excess.”
“If we were 100% sure that what we are seeing is a new particle, that would be huge. That would be revolutionary in physics.”
“Even if the discovery turns out not to be associated with a new particle or dark matter. It would tell us so much more about our Universe, and there would be a lot to learn.”
“We’re not claiming that we’ve made the discovery of the axion yet, but we’re saying that the extra X-ray photons can be explained by axions,” said Dr. Raymond Co, a postdoctoral researcher at the University of Minnesota.
“It is an exciting discovery of the excess in the X-ray photons, and it’s an exciting possibility that’s already consistent with our interpretation of axions.”
Malte Buschmann et al. 2021. Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars. Phys. Rev. Lett 126 (2): 021102; doi: 10.1103/PhysRevLett.126.021102