Searching for photon-ALPs mixing effects in AGN gamma-ray energy spectra

High energy gamma-rays propagating in external magnetic fields may convert into axion-like particles (ALPs). In this case, the observed gamma-ray spectra are modified by the resulting energy-dependent conversion probability. In this study, we use the energy spectra of 20 extra-galactic gamma-ray sources recorded during 10 years of Fermi-LAT observations. We define a test statistics based upon the likelihood ratio to test the hypothesis for a spectral model without vs. a model with photon-ALPs coupling. The conversion probability is calculated for fixed values of the mass and two-photon coupling of the pseudo-scalar particle while the external magnetic field is characterized by the additional free parameters length scale s and average field strength B. As a consistency check and in order to extend the analysis to include very high energy gamma-ray data, another test statistics is defined with the χ² method. We find for 18 of the 20 sources a favorable fit, particularly for Markarian 421 and NGC 1275 a significant improvement, with the hypothesis of photon-ALPs coupling in likelihood analysis. The test statistics of the sources are combined and the significance has been estimated 5.3σ (test statistics summed in local maxima of all sources) and 6.0σ (global maxima). The significance is estimated from dedicated simulations under the null hypotheses. The locally best-fitting values of B and s fall into the range that is expected for large scale magnetic fields present in relevant astrophysical environments.