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  • Letter
  • Open Access

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

Tanmoy Kumar1,*, N. P. S. Mithun2,†, Subhendra Mohanty3,4,‡, Sourov Roy1,§, B. S. Bharath Saiguhan2,∥, and Santosh Vadawale2,¶

  • *Contact author: kumartanmoy1998@gmail.com
  • †Contact author: mithun@prl.res.in
  • ‡Contact author: subhendram@iiserb.ac.in
  • §Contact author: tpsr@iacs.res.in
  • ∥Contact author: bsg@prl.res.in
  • Contact author: santoshv@prl.res.in
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Phys. Rev. D 113, L061303 – Published 16 March, 2026

DOI: https://doi.org/10.1103/3t6n-8xpj

Abstract

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 (gaγγ). Using the latest models of the solar atmosphere to calculate the magnetic field and plasma frequency, we constrain gaγγ≲(0.50–2.26)×10−10  GeV−1 at 95% confidence level for axion masses ma≲5×10−4  eV.

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Phys. Rev. D 113, L061303– Published 16 March, 2026
  • Received 19 November 2025
  • Accepted 11 February 2026
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