Modification of the gamma-ray spectra by internal absorption in optically violently variable blazars: The example cases of 3C 273 and 3C 279

Abstract

ABSTRACT Recent observations with the low-threshold Cherenkov telescopes proved that sub-TeV γ-rays are able to arrive from active galaxies at relatively large distances in spite of the expected severe absorption in the extragalactic background light (EBL). We calculate the γ-ray spectra at TeV energies from two example optically violently variable quasars, 3C 273 and 3C 279, assuming that γ-rays are injected in the inner parts of the jets launched by the accretion discs. It is assumed that γ-rays in the broad energy range (from MeV up to TeV) are produced in these blazars with a power-law spectrum with the spectral index as observed from these objects by the EGRET telescope at GeV energies. We take into account the internal absorption of these γ-rays by considering a number of models for the radiation field surrounding the jet. The classical picture of a relativistic blob in a jet for the injection of primary γ-rays is considered, with the injection rate of γ-rays as observed by the EGRET telescope in the GeV energy range. The results of calculations are compared with positive detection and the upper limits on the sub-TeV γ-ray fluxes from these two sources. It is concluded that, even with the Stecker EBL model, the level of γ-ray emission from 3C 279 is close to the recent measurements in the sub-TeV γ-ray energies, provided that the injected γ-ray spectrum extends from the GeV energies over the next two decades with this same spectral index. We also suggest that a flare with a time-scale of a few days from 3C 273 could be detected by the MAGIC II stereo telescopes. © 2008 RAS.