NuSTAR as an Axion Helioscope

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 gaγ≲7.3×10−12 GeV−1 at 95% CL for axion masses ma≲4×10−7 eV. This constraint surpasses current ground-based experimental limits, studying previously unexplored regions of the axion-photon coupling parameter space up to masses of ma≲3.4×10−4 eV. These findings mark a significant advancement in our ability to probe axion properties and strengthen indirect searches for dark matter candidates.