We propose a unified classification for BL Lac objects (BLs), focusing on the synchrotron peak frequency νs of the spectral energy distribution. The unification scheme is based on the angle Θ that describes the orientation of the relativistic jet and on the electron kinetic luminosity Λkin of the jet. We assume that Λkin scales with the size of the jet r in a self-similar fashion (Λkin ∝ r2), as supported by observational data. The jets are self-similar in geometry and have the same pressure and median magnetic field at the inlet, independent of size. The self-similarity is broken for the highest energy electrons, which radiate mainly at high frequencies, since for large sources they suffer more severe radiative energy losses over a given fraction of the jet length. We calculate the optically thin synchrotron spectrum using an accelerating inner jet model based on simple relativistic gasdynamics and show that it can fit the observed infrared-to-X-ray spectrum of PKS 2155-304. We couple the accelerating jet model to the unification scheme and compare the results to complete samples of BLs. The negative apparent evolution of X-ray-selected BLs is explained as a result of positive evolution of the jet electron kinetic luminosity Λkin. We review observational arguments in favor of the existence of scaled-down accretion disks and broad emission-line regions in BLs. The proposed unification scheme can explain the lack of observed broad emission lines in X-ray-selected BLs, as well as the existence of those lines preferentially in luminous radio-selected BLs. Finally, we review observational arguments that suggest the extension of this unification scheme to all blazars.
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