Searching for axions by analyzing X-ray observations of entire galaxies

The ongoing search for dark matter, the elusive type of matter that does not emit, absorb or reflect light and is estimated to account for most of the universe’s mass, has not yet yielded conclusive results. Axions, hypothetical elementary particles that were first theorized about in the 1970s, are among the most promising candidates for dark matter.

Two different research groups, one based at Università di Padova, Universidad de Zaragoza/TU Dortmund, Deutsches Elektronen-Synchrotron DESY and the SLAC National Accelerator Laboratory and the other at University of California, Berkeley (UC Berkeley), recently performed independent searches for axions using X-ray images of galaxies collected by the Nuclear Spectroscopic Telescope Array (NuSTAR).

Their papers, both of which are published in Physical Review Letters, set even stricter constraints on the properties of axions, while also opening new possibilities for the future search for these elusive dark matter candidates.

“While it is way too early to make a final judgment, it is noteworthy that searches at LHC and at weakly interacting massive particle direct detection experiments reported null results, therefore boosting the ever growing interest in novel sub-GeV particles that couple very feebly to the ordinary matter,” Edoardo Vitagliano, co-author of the first paper, told Phys.org.

“In this regard, axions are some of the best candidates at the moment for physics beyond the standard model. They are a generalization of the QCD axion, a new particle originally proposed in the late 1970s to solve the so-called strong-CP problem.”

In their recent paper, Vitagliano and his colleagues set out to assess the potential of a brand-new probe for conducting axion searches, which is specifically aimed at detecting their interactions with photons. Stars are known to generate a large quantity of light particles; for example, a stellar plasma constantly produces neutrinos, which can easily escape due to their weak interactions with other particles. The idea behind the team’s study is that axions can be produced during similar processes in which photons might weakly interact with them.

“Massive stars within starburst galaxies such as M82 (the Cigar galaxy), in which stars are formed at an exceptionally high rate, can copiously produce axions that exit from the dense cores and subsequently decay outside of the stars,” said Vitagliano. “Since the temperature in such massive stars can reach 100 millions Kelvin, the energy with which axions are shot from the cores can be around 100 keV—thus producing photons similar to the ones used for diagnostic radiography.”

The researchers theorized that the photon flux produced by massive stars would be significantly different from standard X-ray signals originating from the same starburst galaxies. Specifically, they would exhibit a hard spectrum (i.e., dominated by high-energy particles) and angular spread (i.e., with the particles traveling in different directions and over a range of angles), induced by slow particle decays.

These different X-ray signals could thus be a valuable probe for axions. As part of their study, Vitagliano and his colleagues demonstrated this idea using X-ray images collected by the NuSTAR telescope, showing that it could be a promising tool for discovering new decaying particles and exploring physics beyond the Standard Model.

“The axions produced in the core of M82 propagate for tens of thousands of years before finally decaying into photons, producing a halo of X-rays surrounding the galaxy,” explained Damiano Fiorillo, co-author of the paper.

“We have determined the shape and intensity of this halo, and by analyzing the data of the NuSTAR telescope we have found no evidence for this signal among the over 1 Ms of observations. Therefore, the strength with which the axion interacts with the photon must be sufficiently small, and we have used this argument to obtain leading constraints on these feebly interacting particles.”

While the recent search performed by Vitagliano, Fiorillo and their colleagues did not lead to the observation of axions, it allowed them to explore previously uncharted parts of the parameter space for axions with masses below MeV, showing that the M82 galaxy is a powerful laboratory for probing axions with a wide range of masses.

“Tantalizingly, the observation of an axion signal around this mass range with more data would also offer crucial information about the early universe, for example pointing to a relatively low reheating temperature (i.e., the temperature at which inflation ends and one is left with a hot, thermal, radiation-dominated universe),” said Vitagliano.

“Our argument can be applied to other particles beyond the Standard Model and also to different astrophysical sources, as long as the latter are hot enough to produce the new particles, yet cold enough to avoid large boost factors which slow down the decay (due to time dilation, a well-known phenomenon in relativity).”

The findings of the axion search performed by Vitagliano and his colleagues were published around the same time as those of a very similar search, conducted by two researchers at UC Berkeley. This second research team also realized that axions could be probed from NuSTAR observations.

“Our idea started by revisiting the well-known literature that axions can be copiously produced in stars,” said Orion Ning, co-author of the second paper, told Phys.org. “We wanted to play around with the novel idea of how we might be able to maximize the axion signal by predicting the amount of axions produced from the largest stellar populations we could think of—galaxies.

“This simple idea, which has been done at much smaller scales in the past, showed promise in reaching new axion parameter space, especially with judiciously chosen galaxies whose stellar populations can efficiently source axions, such as M82.”

After realizing that M82 and other starburst galaxies could be strong probes for axion physics, Ning and his colleague Benjamin R. Safdi also set out to test this idea. The basis for their search was rooted in what is commonly referred to as an astrophysical “light-shining-through-walls” experiment.

“The first step of this experiment is predicting the axion production inside the stellar interiors of the galaxy’s star population, which comes from photons in the star’s plasma converting to axions,” explained Ning. “These axions then stream out of the star, and as they travel toward us, they can interact with the magnetic fields of the galaxy and then convert back into a photon. These are all mediated by how strongly the axion interacts with electromagnetism, which allows it to then ‘convert’ into photons and vice versa.”

Similarly to Vitagliano and his colleagues, Ning and Safdi also analyzed data collected by the NuSTAR X-ray telescope, searching for the same signatures of axion-induced “light.” While they also did not identify X-ray signatures that could hint at the presence of axions, their efforts set further stringent constraints on the strength of interactions between axions with very small masses (i.e., lighter than 10^-10 eV) and photons.

“Probably the most notable achievement of our work is leveraging this very simple idea—using many, many stars instead of one or a few—to actually maximize the amount of axions we would be able to detect by looking at astrophysical objects,” said Ning. “Excitingly, we’re able to probe a large swath of axion parameter space (mass and coupling) in this work, and we think by opening the door to using entire stellar populations, we will be able make long strides toward finally discovering—or at least narrowing down—where the axion might be.”

Ning and Safdi have now been planning further studies that build on the same idea. In their future research, they plan to perform a similar type of search that instead considers the coupling of axions with electrons, thus focusing on galactic stellar populations that could produce axions originating from electron interactions.

“Similarly, we are currently finishing another variation of this story where axions can be produced by coupling to nucleons (i.e., neutrons and protons), and there are some interesting mechanisms by which this might come about, involving nuclear physics and chemistry,” added Ning.

“There are a variety of other ideas we are playing with as well—the issue is that we don’t know exactly how the axion might show up, so we have to keep exploring all the directions we can think of if we want to definitively discover—or rule out—the axion.”

Vitagliano and his colleagues are also planning additional studies to explore different particle models and probe particles that could remain close to their source due to gravitational attraction, building up to a “basin.” They will try to observe this putative basin around M82, also relying on data from other telescopes, such as INTEGRAL.

© 2025 Science X Network