After the axioms were first theorized by physicists in the suburbs of Chicago 45 years ago, they quickly became a strong candidate for explanation dark matter. All this time, however, the ultra-small particles remain hypothetical. Now a team of astrophysicists speculate that the axions may be responsible for the excess X-ray emissions observed by a group of neutron stars in our galaxy.
The stars – called the “Magnificent Seven” – are neutron stars which emit low-frequency X-rays from their surfaces. Neutron stars are the extremely dense secondary life of collapsed stars. They have powerful magnetic fields and, as their name suggests, are largely composed of neutrons. The new study, published this week in Physical Review Letters focuses on the still unexplained bunch of high-frequency X-rays that the seven stars emit.
“It is possible that what we see here is evidence of new physics, evidence of axions that would transform our understanding of nature in a truly vast way that is difficult to convey,” said Benjamin Safdi, a particle physicist. Lawrence Berkeley National Laboratory and lead author of the recent report said in a phone call. “This discovery may come with this document; may come in 500 years. This is how science works and therefore there is no guarantee of immediate satisfaction. “
The main uncertainty about axions revolve around their existence. In other words, there is a consensus among physicists about the properties that these theoretical particles would possess if they existed. One such property is that axions would interact very weakly and rarely with ordinary matter. Instead of scattering matter in the star, the axions would simply escape. Another is that axions can turn into photons in the presence of magnetic fields – such as those around the seven neutron stars.
The researchers compared the possible behavior of axions with a neutrino, an equally small particle (albeit one that has been proven to exist) that rarely interacts with other matter. It is known that neutrons in neutron stars collide and emit neutrinos, which is the main way in which the star cools over time.
The team’s suggestion is that axions can be created in the centers of neutron stars, where it is much hotter and more energetic than the star’s surface. Just as neutrons in this dense, superheated region produce neutrinos through their collisions, so axions can be formed. The difference is that in the presence of a magnetic field, the axion can turn into a photon. The hissing energy of this photon will be detectable in the X-ray spectrum, in particular in the high frequency range. Previous data were collected for these high-frequency waves, but only as a by-product of the main object of study: low-frequency X-ray waves emanating from the surfaces of stars.
“We are not yet claiming to have discovered the axion, but we say that the additional X-ray photons can be explained by axions,” said Raymond Co., an astrophysicist at the University of Minnesota and co-author of the article, in a press release. “This is an exciting discovery of the excess in X-ray photons, and it’s an exciting opportunity that is already in line with our interpretation of axions.”
Safdi’s hope is that future attention may be drawn to the nearby white dwarf, a degenerate star that is less compact and has a much colder surface temperature than the neutron star. Because white dwarfs do not emit low-frequency X-rays from their surface, no X-ray telescope has ever had much reason to be pointed to one.
“There really is nothing that should appear in every X-ray wavelength,” Safdi said. “If we see a signal, we can be much more confident that what we see is from the axions.”