Evidence from a long-sought hypothetical particle could have been hiding in plain sight (X-ray) all along.
The X-rays emitted from a collection of neutron stars known as the Magnificent Seven is so outrageous that it could come from axions, a long-predicted type of particle forged in the dense cores of these dead objects, scientists have shown.
If their findings are confirmed, this discovery could help unravel some of the mysteries of the physical universe – including the nature of the mysterious dark matter that holds it all together.
“Finding axions has been one of the most important efforts in high-energy particle physics, both in theory and in experiments,” said astronomer Raymond Co of the University of Minnesota.
“We think axions may exist, but we haven’t discovered them yet. You can think of axions as ghost particles. They can exist anywhere in the universe, but they don’t interact strongly with us, so we don’t have any observations of them. yet. “
Axions are hypothetical ultra-low mass particles, first theorized in the 1970s to solve the question of why strong atomic forces follow something called charge parity symmetry, when most models say they don’t need to.
Axions are predicted by many models of string theory – a proposed solution to the tension between general relativity and quantum mechanics – and axions of specific mass are also a strong candidate for dark matter. So scientists have some really good reasons to look for it.
If they exist, axions are expected to be produced in stars. These stellar axions are not the same as dark matter asions, but their existence would imply the existence of other types of axions.
One way to look for axions is to look for excess radiation. Axions are expected to decay in pairs of photons in the presence of a magnetic field – so if more electromagnetic radiation is detected than there should be in an area where this decay is expected to occur, that could be a sign of axions.
In this case, excessive hard X-rays are exactly what astronomers found when viewing the Magnificent Seven.
These neutron stars – the collapsed cores of dead massive stars that died in a supernova – are not clustered in a group, but share a number of features. They are all isolated neutron stars about middle age, a few hundred thousand years ago since death.
They all cool down, emitting low-energy (soft) X-rays. They all have strong magnetic fields, billions of times stronger than Earth’s, powerful enough to cause axion decay. And they are all relatively close, within 1,500 light-years of Earth.
This makes them an excellent laboratory for searching for axions, and when a team of researchers – led by senior author and physicist Benjamin Safdi of Lawrence Berkeley National Laboratory – studied the Magnificent Seven with multiple telescopes, they identified high-energy (hard) X -beam emission not expected for neutron stars of that type.
In space, however, there are many processes that can produce radiation, so the team had to carefully investigate other possible sources of the emission. Pulsars, for example, emit hard X-rays; but the other types of radiation emitted by pulsars, such as radio waves, are not present in the Magnificent Seven.
Another possibility is that undissolved sources near the neutron stars could produce the hard X-rays. But the data sets the team used, from two different X-ray observatories in space – XMM-Newton and Chandra – indicated that the emission came from the neutron stars. Nor, the team found, is the signal likely the result of an accumulation of soft X-rays.
“We are pretty sure this surplus exists, and we are confident that there is something new underneath this surplus,” Safdi said. “If we were 100 percent sure that what we’re seeing is a new particle, that would be huge. That would be revolutionary in physics.”
That does not mean that the surplus is a new particle. It may be a previously unknown astrophysical process. Or it could be something as simple as an artifact from the telescopes or data processing.
“We are not claiming that we have already made the discovery of the axion, but we are saying that the extra X-ray photons can be explained by axions,” said Co. “It’s an exciting discovery of the abundance of X-ray photons, and it’s an exciting possibility that is already consistent with our interpretation of axions.”
The next step is to try to verify the finding. If the surplus is produced by axions, most of the radiation must be emitted with higher energies than XMM-Newton and Chandra can detect. The team hopes to use a newer telescope, NASA’s NuSTAR, to observe the Magnificent Seven over a wider range of wavelengths.
Magnetized white dwarf stars could be another place to look for axion emission. Like the Magnificent Seven, these objects have strong magnetic fields and are not expected to produce harsh X-rays.
“This is starting to be quite convincing that this is slightly out of the standard model if we also see excess X-rays there,” said Safdi.
The research is published in Physical Review Letters.