Physicists show that neutron stars could be enveloped in clouds of axions

A team of physicists from the universities of Amsterdam, Princeton and Oxford have shown that extremely light particles, called axions, can appear in large clouds around neutron stars. These axions could explain the elusive dark matter cosmologists seek, and what’s more, they might not be too difficult to observe.

The research was published in the journal Physical examination and follows earlier work, in which the authors also studied axions and neutron stars, but from a completely different perspective.

While in their previous work they studied the axions that escaped from the neutron star, the researchers now focus on those that remain, the axions that are captured by the star’s gravity. Over time, these particles should gradually form a hazy cloud around the neutron star, and it turns out that such axion clouds may well be observable in our telescopes. But why would astronomers and physicists be so interested in hazy clouds around distant stars?

Axions: From soap to dark matter

Protons, neutrons, electrons, photons: most of us know the names of at least some of these tiny particles. The axion is less known, and for good reason: it is currently only a hypothetical type of particle, which no one has yet detected.

Named after a brand of soap, its existence was first postulated in the 1970s, to solve a problem – hence the reference to soap – in our understanding of one of the particles that we could very well observe: the neutron. However, although theoretically very interesting, if these axions existed they would be extremely light, making them very difficult to detect in experiments or observations.

Today, axions are also known as prime candidates for explaining dark matter, one of the greatest mysteries in contemporary physics. Much evidence suggests that about 85% of the matter in our universe is “dark,” which simply means that it is not made of any type of matter that we currently know and can observe.

Instead, the existence of dark matter is only inferred indirectly through the gravitational influence it exerts 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 is necessarily tiny. As the name suggests, any viable candidate for dark matter is therefore incredibly difficult to observe directly.

Putting them together, physicists realized that the axion could be exactly what they are looking for to solve the dark matter problem. A particle that has not yet been observed, which would be extremely light, and would have very weak interactions with other particles… could axions at least partly explain dark matter?

Neutron stars as magnifying glasses

The idea of ​​the axion as a dark matter particle is interesting, but in physics an idea is only really interesting if it has observable consequences. Would there be a way to observe axions after all, fifty years after their possible existence was first proposed?

When exposed to electric and magnetic fields, axions should be able to convert into photons (particles of light) and vice versa. Light is something we know how to observe, but as mentioned, the corresponding interaction force should be very small, just like the amount of light that axions typically produce. That is, unless we consider an environment containing a truly massive quantity of axions, ideally in very strong electromagnetic fields.

This led researchers to consider neutron stars, the densest stars known in our universe. These objects have masses similar to that of our sun but compressed into stars of 12 to 15 kilometers.

Such extreme densities create an equally extreme environment that notably contains enormous magnetic fields, billions of times more powerful than those we find on Earth. Recent research has shown that if axions exist, these magnetic fields allow neutron stars to mass produce these particles near their surfaces.

Those who stay behind

In their previous work, the authors focused on axions that, after production, escaped the star. They calculated the quantities in which these axions would be produced, the trajectories they would follow, and how their conversion to light might lead to a small but potentially observable effect. signal.

This time, they are interested in the axions that fail to escape, those which, despite their tiny mass, are caught by the immense gravity of the neutron star.

Due to the very weak interactions of the axion, these particles will remain present and, on timescales of up to millions of years, they will accumulate around the neutron star. This can lead to the formation of very dense axion clouds around neutron stars, providing incredible new opportunities for axion research.

In their paper, the researchers study the formation, as well as the properties and subsequent evolution of these axion clouds, emphasizing that they should, and in many cases must, exist.

In fact, the authors argue that if axions exist, axion clouds should be generic (for a wide range of axion properties, they should form around most, perhaps even all, neutron stars), they should in general be very dense (possibly forming a density). twenty orders of magnitude greater than local dark matter densities), and as such, they should lead to powerful observational signatures.

These can come in many types, two of which the authors discuss: a continuous signal emitted for much of a neutron star’s life, but also a one-off burst of light at the end of the neutron star’s life. a neutron star, when it stops producing. its electromagnetic radiation. These two signatures could be observed and used to probe the interaction between axions and photons beyond current limits, even using existing radio telescopes.

What’s next?

Although so far no axion clouds have been observed, with the new results we know very precisely what to look for, making a thorough search for axions much more feasible. While the main item on the to-do list is therefore “searching for axion clouds”, the work also opens up several new theoretical avenues to explore.

For one, one of the authors is already involved in follow-up work that studies how axion clouds can change the dynamics of neutron stars themselves. Another important future research direction is numerical modeling of axion clouds: the present paper shows great discovery potential, but more numerical modeling is needed to know even more precisely what to look for and where.

Finally, the current results all concern single neutron stars, but many of these stars appear as components of binaries, sometimes with another neutron star, sometimes with a black hole. Understanding the physics of axion clouds in such systems, and potentially understanding their observational signals, would be very valuable.

Thus, the present work constitutes an important step in a new and exciting research direction. A complete understanding of axion clouds will require complementary efforts from several branches of science, including particle (astro)physics, plasma physics, and observational radio astronomy.

This work opens this new interdisciplinary area with many opportunities for future research.