A new classification and a two-parameter unification of BL Lacertae objects

A new continuous classification system for BL Lacertae (BL Lac) objects is proposed. The peak frequency of the synchrotron component in the $\nu L_{\nu }$ spectrum is used as a classifying parameter. The application of the accelerating inner jet model to complete BL Lac samples suggests that the range of the observed properties may be explained by adopting a description based on two parameters: the jet angle to the line of sight, and the relativistic electron kinetic luminosity.