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.
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.