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NuSTAR as an Axion Helioscope

J. Ruz ^1,2,*, E. Todarello^3,4,5,†, J. K. Vogel ^1,2,‡, F. R. Candón^1,2, M. Giannotti ^2,6, B. Grefenstette⁷, H. S. Hudson ⁸, I. G. Hannah ⁸, I. G. Irastorza ² et al.

C. S. Kim ⁹, M. Regis ^4,5, D. M. Smith ¹⁰, M. Taoso ⁵, and J. Trujillo Bueno ^11,12

¹Fakultät für Physik, Technische Universität Dortmund, Dortmund, D-44221, Germany
²Centro de Astropartículas y Física de Altas Energías, University of Zaragoza, Zaragoza, 50009, Spain
³School of Physics and Astronomy, University of Nottingham, University Park, NG7 2RD, Nottingham, United Kingdom
⁴Dipartimento di Fisica, University of Torino, I-10125, Torino, Italy
⁵Istituto Nazionale di Fisica Nucleare, Sezione di Torino, I-10125, Torino, Italy
⁶Department of Chemistry and Physics, Barry University, Miami Shores, 33161, Florida, USA
⁷California Institute of Technology, Space Radiation Lab, Pasadena, 91125, California, USA
⁸School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, United Kingdom
⁹Department of Physics, University of California Santa Barbara, Santa Barbara, 93106, California, USA
¹⁰Physics Department and Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, 95064, California, USA
¹¹Instituto de Astrofísica de Canarias, La Laguna, 38205, Tenerife, Spain
¹²Consejo Superior de Investigaciones Científicas, E-28006 Madrid, Spain

^*Contact author: Jaime.Ruz@cern.ch
^†Contact author: elisa.todarello@nottingham.ac.uk
^‡Contact author: Julia.Vogel@cern.ch

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Phys. Rev. Lett. 135, 141001 – Published 2 October, 2025

DOI: https://doi.org/10.1103/18sn-hxtb

Abstract

We present a novel approach to investigating axions and axionlike particles by studying their potential conversion into x-rays within the Sun’s atmospheric magnetic field. Utilizing high-sensitivity data from the nuclear spectroscopic telescope array (NuSTAR) collected during the 2020 solar minimum, along with advanced solar atmospheric magnetic field models, we establish a new limit on the axion-photon coupling strength $g_{a γ} ≲ 7.3 \times 10^{- 12} {GeV}^{- 1}$ at 95% CL for axion masses $m_{a} ≲ 4 \times 10^{- 7} eV$ . This constraint surpasses current ground-based experimental limits, studying previously unexplored regions of the axion-photon coupling parameter space up to masses of $m_{a} ≲ 3.4 \times 10^{- 4} eV$ . These findings mark a significant advancement in our ability to probe axion properties and strengthen indirect searches for dark matter candidates.

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Phys. Rev. Lett. 135, 141001– Published 2 October, 2025

Vol. 135, Iss. 14 — 3 October 2025

Received 9 December 2024
Revised 25 June 2025
Accepted 19 August 2025

DOI: https://doi.org/10.1103/18sn-hxtb

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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NuSTAR as an Axion Helioscope

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