Tiny invisible bullets pass straight through the Sun, barely noticing its matter. These are axions — hypothetical particles so light and elusive that they are often compared to ghosts. Physicists suggest that in the star's hot interior, thermal photons (particles of light) could convert into axions, which then fly out and turn back into photons, but this time in the X-ray range. The magnetic field of the solar atmosphere acts like a developer: if axions exist, they would leave a telltale mark on an X-ray image.
Astronomers used data from the NuSTAR space telescope — an instrument that sees very hard X-rays, exactly the kind axions could produce. The observations were made in 2020, when the Sun was particularly quiet: solar minimum, few sunspots and flares. During such periods, the natural X-ray background is minimal, making any suspicious glow easier to spot. But nothing extra was found — the axions did not show up.
This negative result allowed them to calculate upper limits: how strongly an axion could couple to electrons, atomic nuclei, and photons. The numbers obtained—$g_{ae}\cdot g_{a\gamma}\lesssim 1.1\times10^{-24}\,\mathrm{GeV}^{-1}$ and $g_{aN}^{\rm eff}\cdot g_{a\gamma}\lesssim 2.3\times 10^{-19}\,\mathrm{GeV}^{-1}$—sound like gibberish, but behind them lies a simple idea. If axions clung to ordinary matter any stronger, the telescope would have seen them. Since it didn't, the coupling must be weaker than these numbers. These are the tightest limits to date, obtained not in a laboratory but using an entire star.
as written by the authors arXiv:2605.26265
We investigate solar X-ray observations as a probe of axions and axion-like particles. These particles can be produced in the interior of the Sun via the conversion of thermal photons, as well as through processes involving axion-electron and axion-nucleon interactions. The resulting axions can then reconvert into photons in the Sun's atmospheric magnetic field, generating a signal in the X-ray energy range. In this work, we derive new limits on axions using X-ray observations with the Nuclear Spectroscopic Telescope Array (NuSTAR) during the 2020 solar minimum. In the regime where ALP production is dominated by couplings to electrons or nucleons, we obtain bounds on the product of couplings $$g_{ae}\cdot g_{a\gamma}\lesssim 1.1\times10^{-24}\,\rm GeV^{-1}$$ and $$g_{aN}^{\rm eff}\cdot g_{a\gamma}\lesssim 2.3\times 10^{-19}\,\rm GeV^{-1}$$ at 95% CL, for axion masses $$m_a\lesssim10^{-6}\,\rm eV$$. These constraints strongly improve current ground-based experimental limits, establishing solar X-ray observations as a powerful and robust method for axion searches.
NuSTAR is the first telescope capable of focusing hard X-rays. Its mirrors are coated with a special multilayer coating that reflects photons with energies up to 79 keV — almost like in a dentist's office, but it looks at distant galaxies and the Sun.