Search for anomalous features in gamma-ray blazar spectra corrected for the absorption on the extragalactic background light

Abstract

We consider the ensemble of very-high-energy gamma-ray sources observed at distances and energies where a significant absorption of gamma rays is expected due to pair production on the extragalactic background light (EBL). Previous studies indicated that spectra of these sources, upon correction for the absorption, exhibit unusual spectral hardenings which happen precisely at the energies where the correction becomes significant. Here, we address this subject with the most recent clean sample of distant gamma-ray blazars, making use of published results of imaging atmospheric Cerenkov telescopes and of the Fermi-LAT Pass 8 data, supplemented by the newest absorption models and individual measurements of sources' redshifts. We perform a search for spectral breaks at energies corresponding to unit optical depth with respect to the absorption on EBL. These energies are different for distant and nearby objects, and consequently, such features may not be related to intrinsic properties of the sources. While in some spectra such breaks are not seen, hardenings at distance-dependent energies are present in many of them, though the overall statistical significance of the effect is lower than reported in previous studies. The dependence of the break strength on the redshift found earlier is not confirmed in the new analysis.