The axion makes an excellent dark matter candidate. Confirming that axions are real would be a breakthrough for particle physics—and discovery with far-reaching implications for our understanding of the universe’s composition and history.
In 1970, it was hypothesized that the axions might be produced at the core of stars. The particles are then turned into photons in the presence of a magnetic field.
Recently, a collection of neutron stars, known as the Magnificent 7, provided an excellent testbed for the possible presence of axions, as these stars possess powerful magnetic fields, are relatively nearby – within hundreds of light-years – and were only expected to produce low-energy X-rays and ultraviolet light.
In a new study, physicists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) suggests that axions may be the source of unexplained, high-energy X-ray emissions surrounding a group of neutron stars.
Benjamin Safdi, a Divisional Fellow in the Berkeley Lab Physics Division theory group who led a study, said, “They are known to be very ‘boring,'” and in this case, it’s a good thing.”
“If the neutron stars were of a type known as pulsars, they would have an active surface giving off radiation at different wavelengths. This radiation would show up across the electromagnetic spectrum and could drown out this X-ray signature that the researchers had found or would produce radio-frequency signals. But the Magnificent 7 are not pulsars, and no such radio signal was detected. Other common astrophysical explanations don’t seem to hold up to the observations either.”
“If the X-ray excess detected around the Magnificent seven is generated from an object or objects hiding out behind the neutron stars, that likely would have shown up in the datasets that researchers are using from two space satellites: the European Space Agency’s XMM-Newton and NASA’s Chandra X-ray telescopes.”
According to physicists, probably a new, non-axion explanation arises to consider the observed X-ray excess. Although, physicists are expecting that an answer will lie outside of the Standard Model of particle physics. They think that further observations will confirm the origin of the high-energy X-ray signal.
Safdi said, “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.”
Raymond Co, a University of Minnesota postdoctoral researcher who collaborated in the study, said, “We’re not claiming that we’ve discovered the axion yet, but we’re saying that axions can explain the extra X-ray photons. 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.”
Axions are believed to have the same behavior as neutron stars as they both have very slight masses and interact only very rarely and weakly with other matter. Their production might be 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 strong magnetic field surrounding the Magnificent seven stars – billions of times more potent than magnetic fields produced on Earth – could cause exiting axions to convert into the light.
Safdi said, “We used a bank of supercomputers known as the Lawrencium Cluster at Berkeley Lab in the latest work. Without the high-performance supercomputing work, none of this would have been possible. There is a lot of data processing and data analysis that went into this. You have to model the interior of a neutron star to predict how many axions should be produced inside of that star.”
Safdi noted that “as the next step in this research, white dwarf stars would be a prime place to search for axions because they also have powerful magnetic fields and are expected to be “X-ray-free environments.”
“This starts to be pretty compelling that this is something beyond the Standard Model if we see an X-ray excess there, too.”
The study was supported by the U.S. Department of Energy Office of Science Early Career Research Program; Advanced Research Computing and the Leinweber Graduate Fellowship at the University of Michigan, Ann Arbor; the National Science Foundation; the Mainz Institute for Theoretical Physics (MITP) of the Cluster of Excellence PRISMA+; the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG Excellence Cluster Origins; and the CERN Theory department.
- Malte Buschmann et al. Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars. DOI: 10.1103/PhysRevLett.126.021102