| Summary: | <p>Axion-Like Particles (ALPs) are a very well-motivated class of Beyond the Standard Model particles. They can be implemented as a minimal extension of the Standard Model that could solve many mysteries (the Strong CP Problem, Dark Matter, origin of Inflation), and also commonly arise in String Theory compactifications. This motivates the commissioning of dedicated experiments looking for these particles, as well as the utilisation of telescope data to search for their astrophysical effects. Through their coupling to electromagnetism, they will interconvert with photons in the presence of a magnetic field. A promising environment to look for this effect is the intracluster medium of Galaxy Clusters, which can have weak but very extensive magnetic fields. At X-ray energies, the conversion probability of photons to ALPs is periodic in energy, imprinting a quasi-sinusoidal oscillation on the energy spectrum of an object shining through the cluster. This thesis describes an analysis of data taken by the Chandra and XMM-Newton satellites of point sources (such as active galactic nuclei) shining through galaxy clusters. The absences of modulations in the spectra of these objects lead to constraints on the ALP-photon coupling. With this analysis, a previously unexplored region of parameter space is ruled out. This thesis also details simulations that were performed of the capabilities of the Athena X-ray Observatory, due to launch in 2028, and predicts its ability to place further constraints on ALPs.</p>
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