Axions are hypothetical elementary particles, often accounted to dark matter. Axions’s extremely weak interaction with light has eluded its presence for a long time. Axions could solve two major problems in the realm of physics; the identity of dark matter and the strong nuclear force’s puzzling respect for so-called charge–parity symmetry.
As the detection of this elusive particle often relies on their behavior in strong magnetic fields, the researchers at the University of Amsterdam proposed to look at Axions in the intense magnetic fields of rapidly rotating neutron stars called pulsars.
A new study published in the American Physical Society reports that axions can accumulate around neutron stars and form exceptionally dense clouds. The Universe’s most extreme objects can overcome the feeble interaction strength of axion, allowing us to glimpse them.
Axions, being very dim, could only be produced and converted into detectable in the presence of electric and magnetic fields. Neutron star’s massive strength of magnetic field, surpassing billions of times that of any on Earth, could serve as an efficient axions laboratory.
The insights of the study demonstrate that axions that get trapped in a pulsar’s gravitational pull could pile up to form dense clouds. This dense cloud can grow enormous over a period of a thousand years. This could imprint a detectable signature on the pulsar’s radio spectrum.
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“We show that neutron stars can generate a significant number of axions that remain gravitationally bound to the star, leading to the formation of a dense cloud that grows over thousands of years—a process occurring independently of axions contributing to the observed dark-matter density,” says the study.
The lead author Dion Noordhuis and his colleagues calculated that axions with a mass range between 10–9 and 10–4 eV will be gravitationally trapped around pulsars. Their weak interaction with light will enable them to survive till the lifetime of a pulsar, giving them enough time to accumulate as dense clouds.
Noordhuis says that an axion cloud, interacting with photons, could generate a number of sharp lines in the radio spectrum of each pulsar. Still, the author has urged for a deeper understanding of the systematic uncertainties.
“By simulating a population of neutron stars throughout their lifetimes, we explore the formation, properties, and evolution of these axion clouds. Additionally, we identify distinct observational signatures of axion clouds around neutron stars, such as narrow-band radio emission and bright radio transients.”
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Journal Reference:
D. Noordhuis et al., “Axion clouds around neutron stars,” the American Physical Society, 2024; DOI: 10.1103/PhysRevX.14.041015