Evidence of a long-held hypothetical particle could have been hidden in flat (X-ray) view all this time.
Scientists have shown that the X-ray emission from a collection of neutron stars known as the Magnificent Seven is so excessive that it could come from axions, a long-predicted kind of particle , forged in the dense cores of these dead objects.
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 together.
“Finding axons has been one of the great efforts in high-energy particle physics, both in theory and in experiments,” said astronomer Raymond Co. of the University of Minnesota.
“We believe that 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 still’.
Axions are hypothetical 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 it.
Axions are predicted by many string theory models (a proposed solution to the tension between general relativity and quantum mechanics) and axons of a specific mass are also a strong candidate for dark matter. So scientists have a number of good reasons to go look for them.
If they exist, axions within the stars are expected to occur. These stellar axions are not the same as those of dark matter, but their existence would imply the existence of other types of axions.
One way to look for axions is to look for excess radiation. The axions are expected to disintegrate into photon pairs in the presence of a magnetic field, so if more electromagnetic radiation is detected than there should be in a region where this decay is expected to occur, this could constitute evidence of actions.
In this case, the excess hard X-ray is exactly what astronomers have found when looking at the Magnificent Seven.
These neutron stars (the collapsed nuclei of dead massive stars that died in a supernova) are not grouped into one group, but share a number of features in common. They are all isolated neutron stars from about the Middle Ages, a few hundred thousand years since stellar death.
They all cool and emit low energy (soft) X-rays. They all have strong magnetic fields, trillions times stronger than those on Earth, powerful enough to cause the decay of axion. And they are all relatively close, less than 1,500 light-years from Earth.
This makes them an excellent laboratory for axions, and when a team of researchers – led by author and senior physicist Benjamin Safdi of Lawrence Berkeley National Laboratory – studied the Magnificent Seven with multiple telescopes, they identified X d ‘high energy (hard) -Ray emission is not expected for neutron stars of this type.
However, there are many processes in space that can produce radiation, so the team had to carefully examine other potential sources of emission. Pulsars, for example, emit hard X-rays; but the other types of radiation emitted by the pulsars, such as radio waves, are not present in the Magnificent Seven.
Another possibility is that unresolved sources close to neutron stars could produce hard X-ray emission. But the data sets used by the team, from two different space X-ray observatories, XMM-Newton and Chandra, indicated that the emission came from neutron stars. Nor, the team found, is it likely that the signal is the result of an accumulation of soft X-ray emissions.
“We’re pretty sure that this excess exists and we’re sure there’s something new between this excess,” Safdi said. “If we were 100% sure that what we’re seeing is a new particle, it would be huge. That would be revolutionary in physics.”
This is not to say that excess is a new particle. It could be a hitherto unknown astrophysical process. Or it can be something as simple as a telescope artifact or data processing.
“We don’t claim to have made the discovery of the axion, but we do say that the additional X-ray photons can be explained by axions,” Co said “It’s an exciting discovery of the excess of X-ray photons and it’s an exciting possibility that’s already consistent with our interpretation of axons.”
The next step will be to try to verify the finding. If the excess is produced by axions, most of the radiation should be emitted at energies higher than those capable of detecting XMM-Newton and Chandra. 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 the emission of axons. Like the Magnificent Seven, these objects have strong magnetic fields and are not expected to produce a hard X-ray emission.
“This is starting to get pretty convincing that this is something beyond the standard model if we also see an excess of X-rays in it,” Safdi said.
The research has been published in Physical review letters.