Relativistic Beaming and Orientation Effects in BL Lacertae Objects

We use the correlation between the core-to-lobe radio luminosity ratio (R) and the linear size (D) of a sample of BL Lacertae objects to investigate the relativistic beaming and radio source orientation paradigm for high peaked and low-peaked BL Lacs (X-ray and radio selected BL Lacs respectively) and to constrain relativistic beaming model for this extreme class of active galactic nuclei. We show that the R − D distributions of the BL Lac populations contradict blazar orientation sequence, with the X-ray selected BL Lacs (XBLs) being more consistent with the beaming and orientation model. On the premise that Fanaroff-Riley Type I radio galaxies are the unbeamed parent population of these objects, we derive the bulk Lorentz factor of the jets, γ~7 − 20 corresponding to a critical cone angle for optimum boosting, φc of ~ 1° − 4°, while on average, these objects are inclined at 5° − 12° to the line-of-sight. The implications of these results for the blazar unification sequence are discussed.