Limits on the axion-photon coupling from Chandrayaan-2 observations

Axions and axionlike particles have gained immense attention in searches for beyond Standard Model physics. Experiments searching for axions leverage their predicted couplings to Standard Model particles to look for observable signals. Though weak, these couplings allow axions to be produced abundantly in the interiors of stars such as the Sun. Once created, axions can escape the Sun and, while passing through the solar atmosphere, oscillate into photons in the magnetic field, producing x-rays. For the first time, to the best of our knowledge, we use data from the observation of soft x-rays from the quiet Sun during the 2019–2020 solar minimum by the Solar X-ray Monitor, on board India’s Chandrayaan-2 lunar exploration mission, to constrain the coupling of axions to photons ( g a γ γ ). Using the latest models of the solar atmosphere to calculate the magnetic field and plasma frequency, we constrain g a γ γ ≲ ( 0.50 – 2.26 ) × 10 − 10 GeV − 1 at 95% confidence level for axion masses m a ≲ 5 × 10 − 4 eV .