2022MNRAS.516..216C

We re-examine possible dependencies on redshift of the spectral parameters of blazars observed at very-high energies (VHEs) with Imaging Atmospheric Cherenkov telescopes. This is relevant to assess potential effects with the source distance of the photon to axion-like particle mixing that would deeply affect the propagation of VHE photons across the Universe. We focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6 intermediate-peaked) up to redshift z ≃ 0.5, and a small sample of 5 Flat Spectrum Radio Quasars up to z = 1 treated independently to increase the redshift baseline. The 78 independent spectra of these sources are first of all carefully corrected for the gamma-gamma interaction with photons of the Extragalactic Background Light that are responsible for the major redshift-dependent opacity effect. Then, the corrected spectra are fitted with simple power laws to infer the intrinsic spectral indices Γ_em at VHE, to test the assumption that such spectral properties are set by the local rather than the global cosmological environment. We find some systematic anticorrelations with redshift of Γ_em that might indicate, although with low-significance, a spectral anomaly potentially requiring a revision of the photon propagation process. More conclusive tests with higher statistical significance will require the observational improvements offered by the forthcoming new generation of Cherenkov arrays (CTA, ASTRI, LHAASO).