Physicists suggest that neutron stars may be covered by clouds of axions

A group of physicists from the universities of Amsterdam, Princeton and Oxford have shown that very light particles known as axions can appear in large clouds around neutron stars. These axions could provide an explanation for the elusive dark matter that cosmologists are looking for—and what’s more, it might not be too hard to detect.

The research was published in the journal Physical Examination X and it is a continuation of the previous work, in which the authors also studied axions and neutron stars, but from a completely different perspective.

While in their first work they investigated the axions that escape the neutron star, now the researchers are focusing on the rest—the axions that are captured by the star’s gravity. Over time, these particles should gradually form a faint cloud around the neutron star, and it appears that such axion clouds can be seen with our telescopes. But why would astronomers and physicists be interested in such faint clouds surrounding distant stars?

Axions: From soap to dark matter

Protons, neutrons, electrons, photons—most of us are familiar with the names of at least some of these tiny particles. Axion isn’t very well known, and for good reason: it’s currently a hypothetical type of particle—one that no one has seen.

Named after a type of soap, its existence was introduced in the 1970s, to clear up a problem – and therefore a sign of soap – in our understanding of one of the particles we can best see: the neutron . However, even though, in a very good theory, if these axions did exist they would be very faint, making them very difficult to detect in experiments or observations.

Today, axions are also recognized as the leading candidate to explain dark matter, which is one of the great mysteries in modern physics. Many different pieces of evidence suggest that about 85% of the matter in our universe is “dark,” which simply means that it is not made up of any type of matter that we are familiar with. which we can see for now.

Instead, the existence of dark matter is only indirectly explained by the gravitational influence it has on matter. Fortunately, this does not mean that dark matter has no interactions with matter at all, but if such interactions do exist, their strength is very small. As the name suggests, any active dark matter candidate is very difficult to observe directly.

Putting one together, physicists have realized that the axion may be exactly what they are looking for to solve the problem of dark matter. A never-before-seen particle, potentially very light, and with very few interactions with other particles…could axions be part of the dark matter explanation?

Neutron stars are like magnifying glasses

The idea of ​​an axion as dark matter is fun, but in physics the fun idea actually has observable consequences. Would there be a way to see axions after all, fifty years after the possibility was first proposed?

When exposed to electric and magnetic fields, axions are expected to convert to photons—particles of light—and vice versa. Light is something we know how to observe, but as mentioned, the interaction force must be very small, and so is the amount of light that axions produce in general. That is, unless one imagines an environment with a very large number of axions, well in very strong electric fields.

This led researchers to consider neutron stars, the densest stars known in our universe. These objects have the same mass as our sun but are compressed into stars 12 to 15 kilometers across.

Such extremes create an equally extreme environment, which also has an enormous gravitational field, billions of times stronger than any we find on Earth. Recent research has shown that if axions are present, this magnetic field allows neutron stars to produce these particles in abundance near their surface.

The ones that are left behind

In their previous work, the authors focused on the axions after the production escaped the star – they calculated the amount of these axions that would be produced, whether they would follow which ways, and how their change in light can lead to little but can be seen. a sign

This time, they’re thinking about runaway axions—those that, despite their small mass, are caught in the gravitational pull of a neutron star.

Because of the very weak interactions of the axion, these particles will remain there, and over periods of up to millions of years will accumulate around the neutron star. This can lead to the formation of very dense clouds of axions around neutron stars, offering exciting new opportunities for axion research.

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

In fact, the authors argue that if axions exist, axion clouds should be normal (for most types of material axions should form around the size, maybe even and all neutron stars), should generally be very dense (making the potential mass. Twenty orders of magnitude greater than the density of dark space), and because These should lead to strong monitoring signals.

The latter can come in many forms, of which the authors mention two: a continuous signal produced during large parts of the life of a neutron star, but also bursts of light over time one at the end of a neutron star’s life, when it stops producing. its electrical radiation. Both of these signatures could be detected and used to probe the interaction between axions and photons beyond current limits, even using existing radio telescopes.

What’s next?

Until now, no axion clouds have been seen, with the new results we know exactly what we are looking for, making a thorough search for axions more possible. While the top item on your to-do list is “searching for axion clouds,” the task also opens up many new avenues of exploration.

Another thing is that one of the authors is already involved in follow-up work that studies how axion clouds can change the energy of neutron stars themselves. Another important direction for future research is the mathematical modeling of axion clouds: the present paper shows great potential for detection, but a mathematical model is needed to determine exactly what what you want and where you want it.

Finally, the current results are for single neutron stars, but many of these stars appear as binaries—sometimes together with another neutron star, sometimes together and a black hole. Understanding the physics of axion clouds in such processes, and understanding their warning signals, can be very important.

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

This work opens up a new, diverse field of discipline with many opportunities for future research.