文摘
In this paper, we review the predictions of the popular relativistic beaming model for the unification of radio- and X-ray selected BL Lac (RBL and XBL) populations. Using results of recent analyses, we compare the detailed predictions against observational data and find that RBL–XBL orientation sequence is somewhat questionable. However, FSRQs and BL Lac populations can be unified via luminosity evolution with XBLs and RBLs representing different, but not continuous, stages in the evolution of FSRQs. RBLs, XBLs and FSRQs independently show significant anti-correlation (\(r \geq 0.6\)) between core-to-lobe luminosity ratio and extended radio luminosity, suggestive that they are different representatives of a spectral sequence formed by similar underlying physical phenomenon. These results suggest that there are some fundamental differences between the two BL Lac populations, although orientation effect can also be playing a significant role.