Absorption of bound supernova axion-like particles by the neutron star weakens the heating limits on their photon coupling

Deep within the violent heart of a collapsing star, a supernova explosion creates conditions so extreme that they birth particles we have never seen on Earth. Among these hypothetical particles are axion-like particles, tiny, ghostly entities that interact very weakly with ordinary matter. If these particles exist, they are produced in vast numbers inside the newborn neutron star, the incredibly dense remnant left behind after the explosion. Some of these particles are fast enough to escape the star and fly out into the universe, but others are slower. These slower particles cannot escape the intense gravitational pull of the neutron star; instead, they become trapped, orbiting the star in a vast, invisible cloud. For decades, physicists have wondered if this trapped cloud could be detected. The idea was that as these particles eventually decay, they would turn into light, heating up the surface of the old, cooling neutron star. If we could measure this extra heat, we might finally prove these particles exist.

However, a new study by researchers at the National Institute of Technology in Rourkela, India, has fundamentally changed this picture by showing that the star itself acts as a cosmic vacuum cleaner. The researchers realized that previous calculations had overlooked a critical detail: while the trapped particles orbit the star, their paths are not perfectly circular. Many of these orbits dip deep into the star's interior. Because the star is made of incredibly dense matter, any particle that dives inside is almost instantly absorbed and destroyed by the dense nuclear material. The researchers calculated that for a particle to survive for hundreds of millions of years, it must stay in a very specific, wide orbit that never touches the star's surface. This discovery means that the vast majority of the trapped particles are wiped out long before they can decay and heat the star.

To reach this conclusion, the team built a detailed computer simulation of how a newborn neutron star evolves over its first ten seconds of life. They tracked the paths of millions of these hypothetical particles as the star contracted and cooled. They found that the star's gravity pulls the orbits of the particles inward, causing many of them to crash into the star. Once inside, the particles are absorbed by the dense nuclear matter in a matter of minutes or even seconds, a process far faster than the time it takes for them to decay. The only particles that survive are those born in the outer layers of the star with just the right speed and direction to stay in a safe, circular orbit far from the surface. These survivors are rare, carrying only a tiny fraction of the heat that earlier theories had predicted.

This finding forces a major revision of the limits on how these particles interact with light. Previous studies suggested that if these particles existed, they would heat up old neutron stars to temperatures that we should easily detect. Because the new study shows that most particles are absorbed, the heating effect is much weaker than previously thought. Consequently, the rules for what is possible have changed. The researchers found that a small window of possibility still exists where these particles could survive and heat a star, but the conditions are much stricter. The particles must be heavier and interact with light in a very specific way that cancels out other natural effects. Even with these strict conditions, the study shows that the oldest and coldest known neutron star, PSR J2144−3933, still rules out a specific range of particle masses.

The researchers also explored how the size of the final neutron star affects the outcome. They found that the radius of the star is a crucial factor; a slightly larger star allows more particles to survive because their orbits can stay further away from the surface. Their simulations showed that changing the star's radius by just a few kilometers could alter the number of surviving particles by a factor of nearly four. Furthermore, they considered other ways these particles might be created, such as through interactions with pions, a type of particle that exists inside the star. These additional processes actually help more particles survive, widening the range of possible masses where they could exist, but the overall heating remains weak.

Ultimately, this work does not prove that these particles do not exist, but it does prove that they are much harder to find than we hoped. The "heating" signal that astronomers were looking for is likely too faint to be seen with current telescopes because the star absorbs the particles before they can do their work. The study suggests that if these particles are out there, they are hiding in a very narrow range of properties, and finding them will require either much more sensitive telescopes or a different method of detection. The researchers emphasize that their results depend on the specific models of how neutron stars behave, and future observations of these stars could refine the picture even further. For now, the ghostly cloud of particles around a neutron star is not a warm, glowing beacon, but a sparse collection of survivors that have managed to dodge the star's hungry grasp.