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Constraints on Axion-Like Particles from Ultra-High-Energy Observations of 3HWC J1908+063 with HAWC

Using HAWC Observatory data from the galactic source 3HWC J1908+063, this study found no evidence of photon-axion-like particle conversions and established new exclusion limits on axion-like particle masses between $10^{-8}$ and $10^{-6}$ eV and couplings between $10^{-12}$ and $10^{-10}$ GeV $^{-1}$ .

Original authors: R. Alfaro, C. Alvarez, A. Andres, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velazquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. C

Published 2026-01-22

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CC BY 4.0

Original authors: R. Alfaro, C. Alvarez, A. Andres, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velazquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistran, A. Carraminana, F. Carreon, S. Casanova, U. Cotti, J. Cotzomi, E. De la Fuente, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, J. C. Diaz-Velez, T. Ergin, C. Espinoza, N. Fraija, S. Fraija, J. A. Garcia-Gonzalez, F. Garfias, N. Ghosh, A. Gonzalez Munoz, M. M. Gonzalez, J. A. Gonzalez, J. A. Goodman, J. Gyeong, J. P. Harding, S. Hernandez-Cadena, I. Herzog, D. Huang, F. Hueyotl-Zahuantitla, A. Iriarte, S. Kaufmann, D. Kieda, A. Lara, W. H. Lee, J. Lee, H. Leon Vargas, A. L. Longinotti, G. Luis-Raya, K. Malone, O. Martinez, J. Martinez-Castro, H. Martinez-Huerta, J. A. Matthews, P. Miranda-Romagnoli, P. E. Miron-Enriquez, J. A. Morales-Soto, E. Moreno, M. Mostafa, M. Najafi, A. Nayerhoda, L. Nellen, M. U. Nisa, R. Noriega-Papaqui, N. Omodei, E. Ponce, Y. Perez Araujo, E. G. Perez-Perez, A. Pratts, C. D. Rho, A. Rodriguez Parra, D. Rosa-Gonzalez, M. Roth, A. Sandoval, M. Schneider, J. Serna-Franco, A. J. Smith, Y. Son, R. W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, E. Varela, L. Villasenor, X. Wang, Z. Wang, I. J. Watson, H. Wu, S. Yu, X. Zhang, H. Zhou, C. de Leon

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Hunting for Invisible Ghosts

Imagine the universe is filled with invisible "ghosts" called Axion-Like Particles (ALPs). Scientists think these particles might be the mysterious "dark matter" that holds galaxies together, but they are so light and shy that we can't see them directly.

However, these ghosts have a weird superpower: if they run into a strong magnetic field (like the one surrounding our galaxy), they can briefly turn into light (photons) and then turn back into ghosts. It's like a magician swapping a rabbit for a hat and back again, but the "hat" is a beam of light.

The Experiment: The "High-Altitude Water Cherenkov" Observatory

To catch these ghosts, the scientists used a giant telescope called HAWC, sitting high up on a volcano in Mexico. Think of HAWC as a massive pool of water filled with 300 sensitive light sensors. When high-energy cosmic rays or gamma rays hit the atmosphere, they create a shower of particles that splash into the water, creating a flash of light (Cherenkov radiation) that the sensors can see.

The team focused on a specific cosmic "spotlight" in our galaxy called 3HWC J1908+063. This object is a cosmic lighthouse that blasts out incredibly high-energy gamma rays (hundreds of trillions of electron volts).

The Theory: Why Look at High Energies?

The scientists had a hunch: The higher the energy of the light, the more likely it is to turn into an ALP ghost.

Imagine you are throwing a ball at a wall. If you throw it gently, it bounces off. But if you throw it with superhuman force, maybe it passes right through the wall or changes shape. Similarly, the team predicted that if ALPs exist, the highest-energy gamma rays from our cosmic lighthouse should be "missing" or "dimmed" because some of them turned into invisible ALPs on their way to Earth.

They expected to see a dip in the light spectrum, like a missing note in a song, caused by these particles swapping places with photons.

The Method: The "Taste Test"

The team compared two scenarios:

The "Normal" Scenario (Null Hypothesis): The light travels from the source to Earth exactly as it should, with no ghosts involved. The brightness follows a smooth, predictable curve.
The "Ghost" Scenario (Alternative Hypothesis): The light gets dimmed because some photons turned into ALPs and vanished from our view.

They ran a massive statistical "taste test" using computer simulations (called pseudo-experiments) to see how much the data would have to change to prove the ghosts were real. They needed a very strong signal to say, "Yes, we found a ghost," rather than just a random fluctuation.

The Results: No Ghosts Found (Yet)

After analyzing years of data from the HAWC telescope, the result was clear: The light arrived exactly as predicted by the "Normal" scenario.

No Dimming: There was no evidence of the gamma rays disappearing or turning into ALPs.
The Verdict: The data fits the "no ghosts" model perfectly. The "missing note" in the song wasn't missing at all.

What Did They Learn? (Setting the Boundaries)

Even though they didn't find the ghosts, they didn't come home empty-handed. By proving the ghosts weren't there, they drew a map of where the ghosts cannot be.

Think of it like searching for a lost key in a house. You look in the kitchen, the living room, and the bedroom, and you don't find it. You can't say the key doesn't exist, but you can confidently say, "It is definitely not in these three rooms."

The scientists used their data to set strict exclusion limits:

They ruled out ALPs with specific masses and specific strengths of interaction with light within a certain range.
They showed that if these particles exist, they must be even more elusive (or have different properties) than the specific "candidates" they tested.

The Takeaway

This paper is a story of a very careful search. The scientists looked for a specific type of cosmic magic trick (photons turning into invisible particles) using the brightest lights in our galaxy. The magic trick didn't happen. While they didn't find the new particle, they successfully narrowed down the search area, telling future scientists, "Don't look here; the answer isn't in this specific range of properties."

It's a bit like checking a specific lock and finding it empty; you haven't found the treasure, but you've proven it's not in that specific box.

Technical Summary: Constraints on Axion-Like Particles from Ultra-High-Energy Observations of 3HWC J1908+063 with HAWC

Problem and Motivation
Axion-like particles (ALPs) are hypothetical light pseudoscalar particles proposed to solve the strong CP problem in quantum chromodynamics (QCD) and serve as candidates for cold dark matter. A defining characteristic of ALPs is their coupling to electromagnetism, which allows them to oscillate into photons in the presence of magnetic fields. This conversion probability increases with photon energy, suggesting that ultra-high-energy (UHE) gamma-ray sources (tens to hundreds of TeV) are ideal laboratories for detecting ALP-induced spectral distortions.

While previous studies utilizing Fermi-LAT, H.E.S.S., and HAWC data have focused on distant extragalactic sources (e.g., PKS 2155-304), these analyses face significant systematic uncertainties related to modeling the extragalactic background light (EBL) and intergalactic magnetic fields. This paper addresses these limitations by investigating a galactic source, 3HWC J1908+063. Galactic sources offer distinct advantages: the EBL absorption is negligible at energies below PeV due to short propagation distances, and the magnetic field modeling is restricted to the Milky Way, reducing systematic uncertainties. However, the shorter path length in galactic scenarios requires the use of UHE photons to compensate for the reduced conversion probability.

Methodology
The study analyzes data from the High-Altitude Water Cherenkov (HAWC) Observatory for the source 3HWC J1908+063, a bright galactic emitter with a reported distance of approximately 3.2 kpc (associated with a pulsar wind nebula) and potential associations with a supernova remnant at greater distances.

Spectral Modeling: The authors compare two spectral models fitted to the observed HAWC data:
- Null Hypothesis: A log-parabola model representing the intrinsic source spectrum without ALP conversions.
- Alternative Hypothesis: A log-parabola model modified by the photon-ALP conversion probability ( $P_{\gamma \to a}$ ), which depends on the ALP mass ( $m_a$ ), the photon-ALP coupling constant ( $g_{a\gamma}$ ), the magnetic field strength ( $B$ ), and the propagation distance ( $L$ ).
Statistical Framework: The analysis employs the Multi-Mission Maximum Likelihood Framework (3ML) and the HAWC Accelerated Likelihood plugin (HAL). A test statistic (TS) is defined as the logarithmic likelihood ratio between the null and alternative hypotheses.
- Handling Non-Nested Hypotheses: The authors note that standard Wilks' theorem is inapplicable because the null and alternative hypotheses are not strictly nested (specifically, the case where $m_a = 0$ leads to a degenerate solution for $g_{a\gamma}$ ). Consequently, the TS distribution does not follow a standard $\chi^2$ distribution with 2 degrees of freedom.
- Pseudo-Experiments: To derive unbiased upper limits, the authors generated 300 pseudo-experiments fluctuating the expected counts under the null hypothesis. This allowed them to determine the empirical TS distribution, which was found to be well-described by a $\chi^2$ distribution with 2.8 effective degrees of freedom. This yielded a 95% confidence level (C.L.) TS threshold of 7.5.
Parameter Scenarios: The analysis explores a parameter space of $10^{-8} \text{ eV} \leq m_a \leq 10^{-6} \text{ eV}$ and $10^{-12} \text{ GeV}^{-1} \leq g_{a\gamma} \leq 10^{-10} \text{ GeV}^{-1}$ . To assess systematic uncertainties, the authors varied the assumed distance (3.2 kpc, 5 kpc, 8 kpc) and magnetic field strength (1 $\mu$ G, 3 $\mu$ G), utilizing a homogeneous field approximation consistent with the solar neighborhood and the Galactic plane.

Key Results

Null Detection: The observed data for 3HWC J1908+063 is consistent with the null hypothesis. The observed TS value is 4.39, corresponding to approximately 80% C.L., indicating no statistically significant evidence of photon-ALP conversions. The intrinsic spectrum is well-described by a log-parabola model.
Exclusion Limits: In the absence of a detection, the study establishes exclusion limits on the ALP parameter space at 95% C.L.
- Mass Range: $10^{-8} \text{ eV} \leq m_a \leq 10^{-6} \text{ eV}$ .
- Coupling Range: $10^{-12} \text{ GeV}^{-1} \leq g_{a\gamma} \leq 10^{-10} \text{ GeV}^{-1}$ .
Parameter Sensitivity: The exclusion region is highly sensitive to the assumed magnetic field strength and distance. The exclusion area expands with higher magnetic fields and greater distances, consistent with the conversion probability being proportional to $B_T^2$ and increasing with path length. The analysis shows that for a fixed coupling, increasing the ALP mass significantly raises the critical energy required for conversion, often pushing it beyond the current observational energy range of HAWC.
Comparison: The derived constraints are competitive with existing limits from CAST, H.E.S.S., Fermi-LAT, and magnetic white dwarf polarization measurements, particularly in the specific mass-coupling regime probed by UHE galactic sources.

Significance and Claims
The paper claims that this work demonstrates the utility of UHE observations of galactic sources for probing ALP properties. By utilizing a nearby galactic source, the study minimizes systematic uncertainties associated with extragalactic propagation (such as EBL and intergalactic magnetic fields) that plague previous analyses. The authors emphasize that their methodology correctly accounts for the non-nested nature of the hypotheses, avoiding the under-coverage issues associated with applying standard Wilks' theorem to this specific physical problem.

While no evidence for ALPs was found, the study successfully places competitive constraints on the ALP parameter space, specifically excluding certain combinations of mass and coupling constants. The authors conclude that future observations with more sensitive instruments and refined models could further tighten these limits or potentially reveal ALP-induced phenomena, reinforcing the role of UHE gamma-ray astronomy in the search for dark matter candidates.