X-rays surrounding “Magnificent 7” may be traces of theorized particles

The researchers say they may have found evidence of theorized axions, and possibly dark matter, around a cluster of neutron stars.

A new study, led by a theoretical physicist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), suggests never-before-seen particles called axions may be the source of unexplained high-energy x-ray emissions surrounding a group of neutron stars.

First theorized in the 1970s as part of a solution to a fundamental problem in particle physics, axions are expected to be produced in the hearts of stars and convert into particles of light, called photons, in the presence of a magnetic field.

Axions can also make up dark matter – the mysterious stuff that makes up about 85% of the total mass of the universe, but we have so far only seen its gravitational effects on ordinary matter. Even if the excess x-rays turn out not to be axions or dark matter, it could still reveal new physics.

A collection of neutron stars, known as Magnificent 7, provided an excellent test bed for the possible presence of axions, as these stars have strong magnetic fields, are relatively close – a few hundred light years – and were to produce only weak – energy x-rays and ultraviolet light.

“They’re known to be very ‘boring’,” and in this case that’s a good thing, said Benjamin Safdi, a member of the theory group in the physics division at Berkeley Lab who conducted a study, published on January 12 in the review Physical examination letters, detailing the axial explanation for the excess.

Christopher Dessert, affiliated with the physics division of Berkeley Lab, was a major contributor to the study, which also had input from researchers at UC Berkeley, University of Michigan, Princeton Universityand the University of Minnesota.

If neutron stars were of a type known as pulsars, they would have an active surface emitting radiation at different wavelengths. This radiation would appear across the electromagnetic spectrum, Safdi noted, and could drown out that x-ray signature the researchers had found, or produce radio-frequency signals. But the Magnificent 7 are not pulsars and no radio signal was detected. Other common astrophysical explanations also don’t seem to relate to the observations, Safdi said.

If the excess x-rays detected around Magnificent 7 are generated from one or more objects hidden behind neutron stars, it would likely have manifested in the datasets researchers are using from two space satellites. : the XMM-Newton of the European Space Agency. and NASAChandra X-ray telescopes.

Safdi et al say it is still quite possible that a new non-axial explanation will arise to explain the observed excess x-rays, although they remain hopeful that such an explanation will lie outside the standard model of particle physics, and this new terrain – and space experiments will confirm the origin of the high-energy x-ray signal.

“We are quite confident that this excess exists, and very confident that there is something new about this excess,” Safdi said. “If we were 100% sure that what we are seeing is a new particle, that would be huge. It would be revolutionary in physics. Even though the discovery turns out not to be associated with a new particle or dark matter, he said, “This would tell us so much more about our universe, and there would be a lot to learn.

Raymond Co, a University of Minnesota postdoctoral researcher who collaborated on the study, said: “We are not yet claiming to have made the discovery of the axion, but we are saying that additional x-ray photons may be explained. by axions. This is an exciting discovery of the excess of x-ray photons, and it’s an exciting possibility that is already consistent with our interpretation of axions.

If axions exist, they are expected to behave much like neutrinos on a star, as both are said to have very light masses and only very rarely and weakly interact with other matter. They could be produced in abundance inside the stars. Uncharged particles called neutrons move through neutron stars, occasionally interacting by scattering over each other and releasing a neutrino or perhaps an axion. The neutrino-emitting process is the primary way neutron stars cool down over time.

Like neutrinos, axions could travel outside the star. The incredibly strong magnetic field surrounding the magnificent 7 stars – billions of times stronger than magnetic fields that can be produced on Earth – could cause outgoing axions to be converted into light.

Neutron stars are incredibly exotic objects, and Safdi noted that a lot of modeling, data analysis, and theoretical work went into the latest study. Researchers have made extensive use of a bank of supercomputers known as the Lawrencium Cluster at Berkeley Lab in the latest work.

Some of this work had been done at the University of Michigan, where Safdi previously worked. “Without the high-performance computing work in Michigan and Berkeley, none of this would have been possible,” he said.

“There’s a lot of data processing and analysis that went into all of this. You need to model the interior of a neutron star in order to predict how many axions should be produced inside that star. “

Safdi noted that as a next step in this research, white dwarf The stars would be a great place to look for axions, as they also have very strong magnetic fields and are said to be “x-ray free environments”.

“It starts to be pretty convincing that it’s something beyond the standard model if we see excess x-rays there as well,” he said.

The researchers could also use another X-ray space telescope, called NuStar, to help solve the mystery of the excess X-rays.

Safdi said he was also excited about ground experiments such as CAST at CERN, which functions as a solar telescope to detect axions converted to x-rays by a strong magnet, and ALPS II in Germany, which would use a strong magnetic field to cause axions to turn into particles of light on one side of a barrier like a laser the light hits the other side of the barrier.

Axions have received more attention because a succession of experiments failed to reveal signs of WIMP (massive weakly interacting particle), another promising dark matter candidate. And the image of the axion isn’t that simple – it could actually be a family album.

There could be hundreds of axion-like particles, or ALPs, that make up dark matter, and string theory – a candidate theory for describing forces in the universe – keeps open the possible existence of many types of ALP.

Reference: “Axion The show may explain a new excess of hard X-rays from nearby isolated neutron stars ”by Malte Buschmann, Raymond T. Co, Christopher Dessert and Benjamin R. Safdi, January 12, 2021, Physical examination letters.
DOI: 10.1103 / PhysRevLett.126.021102

The study was supported by the US Department of Energy Office of Science Early Career Research Program; Advanced Research Computing and the Leinweber Graduate Scholarship at the University of Michigan, Ann Arbor; the National Science Foundation; Mainz Institute of Theoretical Physics (MITP) of the PRISMA + Cluster of Excellence; the Munich Institute for Astro and Particle Physics (MIAPP) of the DFG Excellence Cluster Origins; and the CERN Theory Department.