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We discuss our recent constraints on the coupling of Very-Light Axion-LikeParticles (of masses $<$$ 10^{-12} \ \mathrm{eV}$) to electromagnetism from$Chandra$ observations of the cluster-hosted Active Galactic Nuclei (AGN)H1821+643 and NGC1275. In both cases, the inferred high-quality AGN spectraexcluded all photon-ALP couplings $g_\mathrm{a\gamma} > (6.3 - 8.0) \times10^{-13} \ {\mathrm{GeV}}^{-1}$ at the $99.7\%$ level, respectively, based onthe non-detection of spectral distortions attributed to photon-ALPinter-conversion along the cluster line-of-sight. Finally, we present theprospects of tightening current bounds on such ALPs by up to a factor of 10with next-generation X-ray observatories such as $Athena$, $AXIS$ and $LEM$given their improved spectral and spatial resolution and collecting areacompared to current missions.

Current and Future constraints on Very-Light Axion-Like Particles from X-ray observations of cluster-hosted Active Galaxies