Mysterious ‘ghost particle’ may have been spotted in rays from dead star

A mysterious “ghost” particle that scientists have been hunting for decades may have been found in X-rays beaming from dead stars.

The “axion” particle could help scientists unravel some of the mysteries of the universe, including that of “dark matter”, a study argues.

Researchers believe a mysterious, inexplicable increase in X-rays emitted from several neutron stars, which are “dead” stars made up mostly of neutrons, could be a hint of axions.

The study was published in Physical Review Letters.

Raymond Co, a postdoctoral researcher at the University of Minnesota and formerly of the University of Michigan, said: “Finding axions has been one of the major efforts in high-energy particle physics, both in theory and in experiments.

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“We think axions could exist, but we haven’t discovered them yet. You can think of axions as ghost particles. They can be anywhere in the universe, but they don’t interact strongly with us so we don’t have any observations of them yet.”

The researchers believe “extra” X-rays from a collection of neutron stars known as the Magnificent Seven may be caused by axions being produced in the neutron stars’ cores.

The study used a previously proposed theory that axions are produced in the core of a neutron star as byproducts of colliding neutrons and protons.

The particles are said to then shoot out into the star’s strong magnetic field, where they are converted into photons (light particles) that make up the X-rays detected by telescopes on Earth.

Since axions carry much more energy than the photons these neutron stars typically emit, the photons produced from the axions would yield more energy as well, explaining the unexpected increase in X-rays.

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The discovery of axions would answer many questions about dark matter and other particle physics mysteries.

Axions are also predicted by string theory, or the idea that all the forces and particles in the universe are tied together as part of the same framework.

“The axion was first proposed in the late 1970s to solve this problem called the strong CP problem, which means the negative and positive electric charge distributions inside the neutron are centred around the same point,” said Christopher Dessert, a co-author of the paper and graduate student in physics at U-M.

“In the next decade, it was discovered that if the axion existed, it could also be dark matter.”

Axions have been elusive because they are “weakly interacting”, which means they rarely collide with other particles and instead often pass through them.

“We spent an enormous amount of time in that first paper checking to make sure the signal we were seeing is real, and we’re very, very confident that it is,” Dessert said.

“As far as whether it’s an axion – it could be a new astrophysical process in the neutron star we haven’t thought of. But it’s certainly consistent with the existence of an axion making this signal.”

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One way to determine the source of the signal could be to use data from a newer X-ray telescope, Nasa’s NuSTAR, or Nuclear Spectroscopic Telescope Array, rather than older Chandra and XMM-Newton telescopes.

“If it turns out an axion is causing this signal, the Chandra and XMM-Newton telescopes would only actually see a small amount of the signal that would be coming from the axion,” Dessert said.

“A lot of it would be happening at even higher energies than Chandra and XMM-Newton can look at. If we got data from NuSTAR, we could say more confidently: ‘What are the properties of this axion, if it is indeed causing the signal?'”

In addition to Dessert, Co and Safdi, members of the research team include Malte Buschmann, a postdoctoral researcher at Princeton University. The research was funded by grants from the US Department of Energy’s Office of Science and supported by Advanced Research Computing at the University of Michigan.

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