How the Core Composition of Neutron Stars Affects the Axion Mass Bound ✦
Context
Neutron stars are cosmic crucibles where matter is compressed to densities exceeding nuclear density. First proposed by Fritz Zwicky as supernova remnants, they were discovered as pulsars by Jocelyn Bell Burnell. Today these objects serve as laboratories for testing fundamental physics. In their interiors, hypothetical axions—candidates for dark matter—can be born, capable of cooling the star faster than usual. Previously, the axion mass was constrained by observations of supernovae and red giants, but the limit from neutron stars is now the strongest. By comparing theoretical cooling curves with observed temperatures obtained through photometry in the X-ray range, physicists place stringent limits. However, until now these calculations assumed the core consists only of neutrons, protons, and light leptons, ignoring the possibility of strange and heavy baryons appearing. The new work fills this gap, testing the universality of the axion bound.
Methods
Using the modular platform MUSES CE (Modular Unified Solver of the Equation of State), the authors constructed eight equations of state for neutron stars with different core compositions: from standard (neutrons, protons, electrons, muons) to those rich in hyperons and delta resonances. These equations account for spacetime curvature effects and data from gravitational waves from neutron star mergers. Particle production processes increase the rate of entropy removal from the star, accelerating its cooling. Neutron star masses are capped by a limit tracing back to the work of Subrahmanyan Chandrasekhar on degenerate gas. Using the modified code NSCool, the researchers calculated the thermal evolution of stars taking into account all neutrino and axion processes, including recently added channels with hyperons. The analysis included five real neutron stars with known ages and luminosities, and statistical treatment allowed model uncertainties to be accounted for.
Results
It turned out that including hyperons and resonances barely shifts the upper bound on the axion mass. For the KSVZ (Kim–Shifman–Vainshtein–Zakharov) model, the bound remains in the range of 7.6–18.1 meV (depending on the superfluidity model), and for the DFSZ (Dine–Fischler–Srednicki–Zhitnitsky) model — about 15–30 meV. The strongest limit, obtained for a composition without exotica and with suppressed neutrino emission, turned out to be almost twice as tight as previous estimates and falls within the sensitivity range of the future helioscope IAXO. Interestingly, the best agreement with observations is achieved for zero axion mass, indicating a good understanding of standard cooling mechanisms. These results demonstrate for the first time that the axion mass bound is robust against variations in core composition, confirming its reliability as a key benchmark for particle physics.
Implications
The stability of the axion bound with respect to core composition is great news for physicists: it means the limit is not an artifact of a simplified model. Moreover, if the axion is discovered within this window, its precise mass could reveal whether hyperons or other exotic particles exist in neutron star cores. This turns the axion from a hypothetical particle into a tool for "neutron tomography," capable of peering into the very heart of ultra-dense matter. The work also highlights the importance of an interdisciplinary approach: bringing together nuclear physics, astrophysics, and particle physics.
Future development
Next steps include studying even more exotic compositions, such as those with quark-gluon plasma (hybrid stars). The MUSES CE platform already allows modeling the phase transition from hadronic matter to quark matter, opening the way to calculations of axion production in such environments. Additionally, future observations from space observatories like James Webb and next-generation X-ray telescopes will refine the temperatures and ages of isolated neutron stars, increasing the precision of the constraints. Theorists will also need to account for pion condensate effects and describe axion emission in exotic phases in more detail.
Impact
The results directly influence axion search strategies (experiments like IAXO) and our understanding of the nature of dark matter. They are also important for interpreting data from gravitational wave events and for refining the equation of state of ultra-dense matter.
Next steps
Incorporate quark matter into simulations and study the cooling of hybrid stars with axion cooling. Perform joint analysis with new observational data, including from NICER and future missions.
Key open problems
The work connects three fundamental problems: the strong CP problem (axions as its solution), the nature of dark matter, and the equation of state of neutron stars. The uncertainty in core composition is one of the key challenges of high-density astrophysics, and the axion window may hold the key to the puzzle.
🎯 The density at the center of a neutron star is such that a teaspoon of its material would weigh as much as Mount Everest. And the axion, if discovered, would be the first particle "seen" not by its radiation but by its indirect thermal footprint in stellar remnants.
🎬 In science fiction, neutron stars often serve as exotic settings: for example, Robert Forward's novel "Dragon's Egg" describes intelligent life on the surface of an ultra-dense star, and Larry Niven's story "Neutron Star" features monstrous tidal forces.
Key numbers
- KSVZ axion mass bound: 7.6–18.1 meV
- DFSZ axion mass bound: ≈15–30 meV
- typical neutron star luminosity: ~10^{32}–10^{33} erg/s
- age of stars in the sample: 0.35–0.85 million years
- core density: up to 10^{15} g/cm^3