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

We discuss our recent constraints on the coupling of Very-Light Axion-Like Particles (of masses $<$${10}^{-12}\mathrm{e}\mathrm{V}$) 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 spectra excluded all photon-ALP couplings $g_{\mathrm{a}\gamma }>(6.3-8.0)\times {10}^{-13}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ at the $99.7\mathrm{\%}$ level, respectively, based on the non-detection of spectral distortions attributed to photon-ALP inter-conversion along the cluster line-of-sight. Finally, we present the prospects of tightening current bounds on such ALPs by up to a factor of 10 with next-generation X-ray observatories such as $Athena$, $AXIS$ and $LEM$ given their improved spectral and spatial resolution and collecting area compared to current missions.