Unexplained X-ray signals from neutron stars hint at "ghost" particles
Astronomers have detected a strange signal coming from a group of neutron stars that could be the fingerprints of a long-sought elementary particle – and maybe even dark matter. An unexplained excess of X-rays hints at axions, hypothetical “ghost” particles that could solve several long-standing physics puzzles.
When massive stars go supernova, they can leave behind neutron stars – extremely dense cores with powerful magnetic fields. A group of neutron stars nicknamed the Magnificent Seven was expected to produce ultraviolet light and low-energy X-rays, but a few years ago astronomers discovered they were also somehow giving off high-energy X-rays, which can’t be explained by current models.
In the new study, the team proposed an intriguing explanation – the signals could be caused by axions. These hypothetical particles were first proposed in 1977 to solve a cosmological conundrum called the strong CP problem, which (put very simply) ponders why neutrons don’t interact with electric fields. Later, axions emerged as a promising candidate for dark matter, the mysterious substance believed to outnumber regular matter particles by a ratio of five to one.
If they exist, axions would have very small masses and rarely interact with regular matter, zipping through people and planets unimpeded – hence the “ghost particle” moniker. But one way they might make their presence known is by interacting with electromagnetic fields, such as those surrounding neutron stars.
The researchers propose that axions could be created in huge numbers in the heart of these neutron stars and make their way to the outside, where the magnetic field would convert them into photons. Since axions carry large amounts of energy, their photons would too, potentially producing the high-energy X-rays observed around these stars.
“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,” says Raymond Co, an author of the study. “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.”
Interestingly, astronomers have already run with this interpretation of axions and tried to find the particles by searching for signs of radio signals around neutron stars, or X-ray anomalies in galaxy clusters, to no avail. That makes this new detection of such a strange signal, fitting the bill for axions, incredibly intriguing. Even better, they may have already turned up in lab experiments too.
If axions do turn out to exist, they’d be very handy little particles. Not only could they solve the strong CP problem and dark matter, but they strengthen the case for string theory and may help explain why the universe contains so much more matter than antimatter.
Even if the X-ray excesses aren’t caused by axions, the team says they may still raise questions beyond the scope of the Standard Model, hinting at brand new physics. To find out, the next steps would be to investigate white dwarf stars, which 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,” says Benjamin Safdi, lead author of the study.
The research was published in the journal Physical Review Letters.