Space & Cosmos

Physicists show that neutron stars may be covered in axion clouds

Axion cloud around a neutron star. Although some axions escape the star’s gravity, many remain bound to the star and form clouds that surround it for long periods of time. Interaction with the neutron star’s strong magnetic field converts some axions into photons, or light that can eventually be detected by telescopes on Earth. Credit: University of Amsterdam

A team of physicists from the University of Amsterdam, Princeton University and Oxford University has shown that extremely light particles known as axions can originate in large clouds around neutron stars. These axions could form the explanation for the elusive dark matter that cosmologists are looking for, and what’s more, they might not be all that difficult to observe.

The study, published in the journal Physical Review

Previous research has studied axions that escape from neutron stars, but this time we focus on axions that are left behind, that is, axions that are captured by the star’s gravity. It turns out that over time, these particles should gradually form a faint cloud around the neutron star, and such axion clouds could be well observed with our telescopes. But why are astronomers and physicists so interested in hazy clouds around distant stars?

Axion: From soap to dark matter

Most of us are familiar with the names of at least some of these small particles – protons, neutrons, electrons, photons. Axions are not well known, and for good reason. For now, it’s just a hypothetical type of particle that no one has detected yet.

Named after a brand of soap, its existence was first envisioned in the 1970s to solve a problem in our understanding of one of the particles we often observe: neutrons. Therefore it is called soap. However, while great in theory, these axions are very light when present, making them very difficult to detect experimentally or observationally.

Today, axions are also known as the leading candidate to explain one of the greatest mysteries in modern physics: dark matter. Many different lines of evidence suggest that about 85% of the matter in our universe is “dark.” This simply means that the universe is not made up of any kind of matter that we know and can currently observe.

Instead, the existence of dark matter is inferred only indirectly through its gravitational influence on visible matter. Fortunately, this does not automatically mean that dark matter has no other interactions with visible matter, but if such interactions exist, their strength must necessarily be is small. As the name suggests, directly observing viable dark matter candidates is incredibly difficult.

Putting these one and one together, physicists realized that axions might be exactly what they were looking for to solve the dark matter problem. Particles that have not yet been observed would be very light and have very weak interactions with other particles… Could axions be at least part of the explanation for dark matter?

Neutron star as a magnifying glass

It’s nice to think of axions as dark matter particles, but in physics, an idea is only really great if it has observable consequences. It’s been 50 years since the possible existence of axions was first proposed, but is there a way to observe them after all?

When exposed to electric and magnetic fields, axions are expected to be able to convert into photons (particles of light) and vice versa. Light is something we know how to observe, but as mentioned above, the strength of the corresponding interaction should be very small, and therefore the amount of light that axions generally produce as well. That is, unless you consider an environment with really large amounts of axions, ideally a very strong electromagnetic field.

This led researchers to consider neutron stars, the densest known stars in the universe. These objects have masses similar to the Sun, but are compressed into stars 12 to 15 kilometers in size.

Such extreme densities create an equally extreme environment, which also contains, among other things, a huge magnetic field billions of times stronger than those found on Earth. Recent studies have shown that in the presence of axions, these magnetic fields allow neutron stars to produce large quantities of these particles near their surfaces.

Physicists show that neutron stars may be covered in axion clouds

An overview of the four stages that characterize the formation and evolution of axion clouds around neutron stars. Credit: Physical Review X (2024). DOI: 10.1103/PhysRevX.14.041015

those who remain behind

In previous work, the authors focused on axions that escaped from stars after their formation, and investigated how much these axions are produced, what kind of trajectories they follow, and how their conversion to light is weak but latent. We calculated how it connects to observable stars. signal.

This time, they consider axions that could not escape, that is, axions that are trapped by the neutron star’s enormous gravity despite their small mass.

Due to the very weak interactions of axions, these particles remain in the periphery and accumulate around the neutron star on time scales of up to millions of years. This can lead to the formation of extremely dense axion clouds around neutron stars, providing surprising new opportunities for axion research.

In their paper, the researchers study the formation, properties, and further evolution of these axion clouds and point out that axion clouds should, and in many cases must, exist.

In fact, the authors argue that if axions exist, axion clouds should be common (due to the wide range of axion properties, they should form around most, perhaps all, neutron stars) and that they are generally very It should be dense (and potentially form a density 20 orders of magnitude greater than the local dark matter density), and thus should lead to strong observational signatures.

There may be many types of the latter, two of which the authors discuss. One is a continuous signal emitted during most of a neutron star’s life, but there is also a one-time burst of light at the end of a neutron star’s life, when the neutron star stops forming. Its electromagnetic radiation. Both of these signatures can be observed and used to probe interactions between axions and photons beyond current limits, even using existing radio telescopes.

What’s next?

So far, no axion clouds have been observed, but these new results tell us very precisely what to look for and make a thorough search for axions more feasible. So while a major point on the to-do list is ‘searching for axion clouds’, this work also opens up some new theoretical avenues to explore.

First, one of the authors is already involved in a follow-up study studying how axion clouds change the dynamics of the neutron star itself. Another important future research direction is numerical modeling of axion clouds. This paper shows great potential for discovery, but further numerical modeling is needed to know more precisely what to look for and where to look.

Finally, although the present results all concern single neutron stars, many of these stars appear as components of binaries, sometimes with other neutron stars, and sometimes with black holes. Understanding the physics of axion clouds in such systems and potentially understanding their observational signals would be of great value.

The current study is therefore an important step in a new and exciting research direction. A complete understanding of axion clouds requires complementary efforts from multiple areas of science, including particle (astro)physics, plasma physics, and observational radio astronomy.

This study opens up this new transdisciplinary field and provides many opportunities for future research.

Further information: Dion Noordhuis et al., Axion clouds around neutron stars, Physical Review X (2024). DOI: 10.1103/PhysRevX.14.041015

Provided by University of Amsterdam

Citation: Physicists show that neutron stars may be encased in clouds of axions (October 18, 2024) https://phys.org/news/2024-10-physicists- Retrieved October 18, 2024 from neutron-stars-shrouded-clouds.html

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