We discuss the relation between the power carried by relativistic jets and the nuclear power provided by accretion, for a group of blazars including FSRQs and BL Lac objects. They are characterized by good quality broad band X-ray data provided by the Beppo SAX satellite. The jet powers are estimated using physical parameters determined from uniformly modelling their spectral energy distributions (SEDs). Our analysis indicates that for Flat Spectrum Radio Quasars the total jet power is of the same order as the accretion power. We suggest that blazar jets are likely powered by energy extraction from a rapidly spinning black hole through the magnetic field provided by the accretion disk. FSRQs must have large BH masses (108 − 109M⊙) and high, near Eddington accretion rates. For BL Lac objects the jet luminosity is larger than the disk luminosity. This can be understood within the same scenario if BL Lac objects have masses similar to FSRQ but accrete at largely subcritical rates, whereby the accretion disk radiates inefficiently. Thus the “unification ” of the two classes into a single blazar population, previously proposed on the basis of a spectral sequence governed by luminosity, finds a physical basis. – 3 – 1
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