Speaker
Description
The intrinsic gamma-ray flux from extragalactic sources in the very-high-energy (VHE; E>100GeV) regime is subject to attenuation due to interactions with photons of the extragalactic background light (EBL), leading to pair production. Consequently, the Universe is expected to appear opaque to VHE photons above certain energy thresholds, depending on the redshift of the source. An oscillation of VHE photons into axion-like particles (ALPs) with masses below $\sim 10^{-6}$ eV in the presence of ambient magnetic fields could reduce the opacity, as ALPs can propagate unimpeded over cosmological distances. On the other hand, heavier ALPs with masses around 10 eV that contribute to the dark matter density of the Universe could decay into two photons and contribute to the EBL, thereby increasing the gamma-ray opacity. In this study, we analyze a large sample of gamma-ray spectra obtained with the High Energy Stereoscopic System (H.E.S.S.) and the Fermi-Large Area Telescope to search for these ALP signatures. Two independent approaches are adopted to explore distinct regions of the ALP parameter space. The first approach models ALP-induced spectral upturns in gamma-ray observations, while the second investigates the effect of ALP decay on the EBL and its measurable impact on blazar spectra.
| Collaboration(s) | The H.E.S.S. Collaboration |
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