Astronomy and Astrophysics – Astrophysics
Scientific paper
2000-02-22
Astronomy and Astrophysics
Astrophysics
9 pages in emulateapj.sty, 3 figures. Astrophysical Journal, Vol 538, 20 July Issue, 2000
Scientific paper
10.1086/309105
Photons with very high energy up to TeV (VHE) emitted from active galactic nuclei (AGNs) provide some invaluable information of the origin of $\gam$-ray emission. Although 66 blazars have been detected by {\it EGRET}, only three low redshift X-ray selected BL Lacs (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 ({\it external} absorption). Based on the ``mirror'' effect of clouds in broad line region, we argue that there is an {\it intrinsic} mechanism for the deficiency of TeV emission in blazars. Employing the observable quantities we derive the pair production optical depth $\tgg(\eps_{\rm obs})$ due to the interaction of VHE photons with the reflected synchrotron photons by electron Thomson scattering in broad line region. This sets a more strong constraints on very high energy emission, and provides a sensitive upper limit of Doppler factor of the relativistic bulk motion. It has been suggested to distinguish the {\it intrinsic} absorption from the {\it external} by the observation on variation of multiwavelegenth continuum.
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