Beaming in blazars

Relativistic beaming of radiation is generally invoked to explain the extreme properties of blazars. We have applied the simplest synchrotron–self-Compton (SSC) model to all blazars that have been observed in the X-ray region and have known VLBI sizes, and derived the minimum kinematic Doppler factor for each source. All high polarization quasars (HPQs), but only half of the 30 BL Lac objects in our sample require relativistic enhancement of radiation due to beaming effects. The different Doppler factor distributions for the two classes of sources argue against the hypothesis that BL Lacs could be low-luminosity HPQs whose strong emission lines are swamped out by the boosted non-thermal continuum. Thus HPQs and BL Lacs could be intrinsically different in their physical structure.