Abstract The Königl inhomogeneous jet model can successfully reproduce most observational features of jets in active galactic nuclei (AGN), when suitable physical parameters are adopted. We improve Königl's calculations on the core emission from the jet with a small viewing angle θ0~ ( is half opening angle of the conical jet). The proper motion of the jet component provides a constraint on the jet kinematics. Based on the inhomogeneous jet model, we use the proper motion data of the jet component to calculate the minimal kinetic luminosity of the jet required to reproduce the core emission measured by the very-long-baseline interferometry (VLBI) for a sample of BL Lac objects. Our results show that the minimal kinetic luminosity is slightly higher than the bolometric luminosity for most sources in the sample, which implies that radiatively inefficient accretion flows (RIAFs) may be in those BL Lac objects, or/and the properties of their broad-line regions (BLRs) are significantly different from flat-spectrum radio-loud quasars.
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