A team of physicists has proposed that a hypothetical particle, believed to constitute the universe’s dark matter, may be generated and persist around neutron stars, which are some of the densest objects in the cosmos. The particles, known as axions, are among the candidates considered for dark matter, a mysterious substance comprising over a quarter of the universe’s mass. Researchers from the universities of Amsterdam, Princeton, and Oxford suggest that axions could form clouds around neutron stars, the dense remnants of dead stars. This hypothesis provides a new focus for astrophysical dark matter searches and emphasizes the potential benefits of employing a radio telescope in space.
The scientists speculate that axions generated within neutron stars might transform into photons and escape into space. However, many of these particles could remain trapped by the star’s gravity, forming an axionic cloud. Their research, recently published in Physical Review X, follows earlier work exploring how axions might escape the gravitational fields of neutron stars. Anirudh Prabhu, a research scientist at the Princeton Center for Theoretical Science and co-author of the paper, explained that while light rarely interacts with axions, the Primakoff effect allows axions to convert to light in the presence of a strong magnetic field. Neutron stars with high magnetism, known as magnetars, provide ideal conditions for this conversion, potentially making observable light detectable by space-based telescopes.
Dark matter, which remains undetectable through emitted light and interacts with ordinary matter only through gravitational forces, makes up approximately 27% of the universe. Other proposed candidates for dark matter include Weakly Interacting Massive Particles (WIMPs), dark photons, and primordial black holes. Axions were initially suggested to resolve a particle physics anomaly related to unobserved characteristics of neutrons. Despite the challenges in observing axions, last year’s study on Einstein rings, regions where light bends due to gravity, added to axions’ potential as dark matter candidates. Space-based observatories are better suited to detect axions since Earth’s ionosphere obstructs long wavelengths emitted by these particles.
Benjamin Safdi, a particle physicist at UC Berkeley, acknowledged the potential for axion production in neutron stars, an idea that seems evident in retrospect due to the stars’ strong gravitational forces. He co-authored a 2021 study proposing axion production in the Magnificent Seven, a group of neutron stars, where axion-photon conversion might explain observed X-rays. The current research team suggests that axions remain near their sources, forming dense clouds over extensive periods, and that existing radio telescopes could improve sensitivity to axion-photon coupling.
While advanced telescopes like the Webb Space Telescope and ESA’s Euclid Space Telescope have focused on infrared observations, proposals such as the Lunar Crater Radio Telescope (LCRT) aim to enhance radio-based observatories. Safdi advocates for axions as a promising path for new physics, emphasizing their challenging nature due to weak interactions with regular matter. Extreme environments like neutron star magnetospheres could amplify these interactions, potentially paving the way for discoveries. Despite considerable achievements from Earth-based radio telescopes, a new space-based mission may be necessary to observe axionic waves, highlighting a significant potential avenue for NASA’s future resources.
