Intrinsic constraints on very high energy emission in $\gamma$-ray loud blazars

摘要

Photons with very high energy up to TeV (VHE) emitted from active galacticnuclei (AGNs) provide some invaluable information of the origin of $\gam$-rayemission. Although 66 blazars have been detected by {\it EGRET}, only three lowredshift X-ray selected BL Lacs (Mrk 421, Mrk 501, and 1ES 2344+514) areconclusive TeV emitters (PKS 2155-304 is a potential TeV emitter) since VHEphotons may be absorbed by cosmological background infrared photons ({\itexternal} absorption). Based on the ``mirror'' effect of clouds in broad lineregion, we argue that there is an {\it intrinsic} mechanism for the deficiencyof TeV emission in blazars. Employing the observable quantities we derive thepair production optical depth $\tgg(\eps_{\rm obs})$ due to the interaction ofVHE photons with the reflected synchrotron photons by electron Thomsonscattering in broad line region. This sets a more strong constraints on veryhigh energy emission, and provides a sensitive upper limit of Doppler factor ofthe relativistic bulk motion. It has been suggested to distinguish the {\itintrinsic} absorption from the {\it external} by the observation on variationof multiwavelegenth continuum.