SIMBAD references

Photons with very high energy (VHE) up to TeV emitted from active galactic nuclei (AGNs) provide some invaluable information on the origin of γ-ray emission. Although 66 blazars have been detected by EGRET, only three low-redshift X-ray-selected BL Lac objects (Mrk 421, Mrk 501, and 1ES 2344+514) are conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter), since VHE photons may be absorbed by cosmological background infrared photons (external absorption). Based on the ``mirror'' effect of clouds in the broad-line region, we argue that there is an intrinsic mechanism for the deficiency of TeV emission in blazars. Employing the observable quantities, we derive the pair production optical depth τ_γγ(ε_obs) due to the interaction of VHE photons with the reflected synchrotron photons by electron Thomson scattering in the broad-line region. This sets stronger constraints on very high energy emission, and provides a sensitive upper limit on the Doppler factor of the relativistic bulk motion. It has been suggested that the intrinsic absorption be distinguished from the external by the observation on variation of the multiwavelength continuum.