Probing Axion–Photon Mixing with High-resolution X-Ray Spectroscopy

Axion-like particles (ALPs) provide a compelling avenue for exploring physics beyond the Standard Model. In astrophysical magnetized plasmas, an ALP–photon coupling g aγ induces energy-dependent oscillations in the photon survival probability that imprint modulations on emission spectra. X-ray observations of bright spectrally smooth sources can provide particularly sensitive probes of ultralight ALPs with masses m a ≲ 10 −11 eV due to long propagation distances, strong magnetic fields, and high photon statistics. We present a comprehensive forecast of ALP–photon conversion in three representative systems: (i) background active galactic nuclei (AGNs) observed through foreground intracluster magnetic fields, (ii) central AGNs within their host cluster halos, and (iii) Galactic X-ray binaries viewed through the Milky Way field. Using detailed simulations, we assess the prospective sensitivity of high-resolution X-ray missions including XRISM, Athena, and Arcus. For typical magnetic field configurations, a 320 ks XRISM observation or an 8 ks Athena observation of the Perseus Cluster AGN NGC 1275 can reach down to g aγ ∼ 5 × 10 −13 GeV −1 at m a ≲ 10 −12 eV, surpassing the current best limit. We quantify the impact of magnetic field modeling, photon statistics, and spectral binning strategies. Our results demonstrate the scientific potential of future X-ray observatories to probe photon–ALP coupling in previously inaccessible parameter space, offering a powerful window into physics beyond the Standard Model.