Assessing the sensitivity to Axion-Like-Particle Dark Matter with very-high-energy gamma-ray observations of selected AGN and galaxy cluster pairs
Axion-Like-Particles (ALPs) are pseudo-scalar particles actively searched as light dark matter candidates. ALPs can couple to photons which give rise to the possibility of oscillations with photons in an external magnetic field. If sufficiently strong, this coupling can imprint distinctive spectral irregularities in the gamma ray spectrum of astrophysical sources. We present a prospective study on the sensitivity of probing ALP-photon interactions using stacked observations of selected active galactic nuclei (AGNs) located behind galaxy clusters. The ALP-photon conversion in cluster magnetic fields produces absorption-like features in AGN spectra that are difficult to predict for individual sources. To address this, we apply a stacking analysis of multiple AGN-cluster pairs, yielding a controlled prediction of the expected ALP induced spectral patterns and enhancing the sensitivity to such irregularities. Using simulated data for selected hard-spectrum Fermi/LAT AGNs that can be observed by Imaging Atmospheric Cherenkov Telescopes such as H.E.S.S., we evaluate the performance of this method. The combination of mock IACT observations with our stacking approach enable exploration of the previously uncharted ALP dark matter parameter space in the neV mass range.