Sensitivity to Axion-like Particle dark matter with very-high-energy gamma-ray observations of Active Galactic Nuclei located behind Galaxy Clusters

TL;DR

This study forecasts the sensitivity of gamma-ray observations of AGNs behind galaxy clusters to detect axion-like particles, revealing potential to probe uncharted ALP dark matter parameter space with current and future telescopes.

Contribution

It introduces a stacking analysis method of multiple AGN-cluster pairs to improve sensitivity to ALP-photon interactions in gamma-ray spectra.

Findings

Sensitivity to ALP-photon coupling down to 6×10⁻¹³ GeV⁻¹ at 3×10⁻⁸ eV mass.

Stacking analysis enhances detection prospects for ALPs in the 10⁻⁸ to 10⁻⁷ eV mass range.

Current statistical limitations suggest further improvements with more observations.

Abstract

Axion-Like-Particles (ALPs) are hypothetical pseudo-scalar particles actively searched as light dark matter candidates. The coupling of ALPs to photons can give rise to distinctive spectral features in the observed gamma-ray spectrum of astrophysical sources. We perform a forecast study on the sensitivity to ALP-photon interactions using stacked mock observations of selected active galactic nuclei (AGNs) located behind galaxy clusters (GC). The ALP-photon conversion in the magnetic fields of galaxy clusters give rise to absorption-like features in AGN spectra that are subject to large variance in their prediction for individual sources. We consider here a stacking analysis of multiple AGN-cluster pairs, which yields a more controlled prediction of the expected ALP-induced spectral patterns in the observed gamma-ray spectra. Using realistic mock observations of selected Fermi-LAT AGNs by ongoing Imaging Atmospheric Cherenkov Telescopes such as H.E.S.S., MAGIC and VERITAS, we provide a careful assessment of the expected sensitivity of a combined statistical analysis of many AGN-GC pairs, together with the impact of modelling and instrumental uncertainties. The sensitivity reaches ALP-photon couplings down to 6$\times$10$^{-13}$ GeV$^{-1}$ for an ALP mass of 3$\times$10$^{-8}$ eV, and is currently statistically dominated indicating further improvements from more observations. Such a stacking analysis approach enables exploration of the yet-uncharted ALP dark matter parameter space in the 10$^{-8}$ - 10$^{-7}$ eV mass range.