Abstract We calculate the maximal jet power extractable from a rapidly spinning black hole or the accretion disk (standard thin disk or advection dominated accretion flow, ADAF) surrounding the black hole. Compared the theoretical calculations with the observational data, ADAFs are required to be present in the inner regions of the disks and transit to standard thin disks at radii of ~ 40–150 GMbh/c2 for BL Lac objects. For some radio quasars, their jets are too powerful to be extracted from the standard thin accretion disks or rapidly spinning black holes surrounded by standard thin disks. If the ADAFs are present in these quasars, their bright optical continuum luminosity cannot be produced by pure-ADAFs due to their low accretion rates and low radiation efficiency, unless the dimensionless accretion rates = /Edd can be as high as 0.05 and the ADAFs transit to standard thin disks at rather small radii of ~ 20GMbh/c2. We propose that the disk-corona structure is present at least in some radio quasars. The plasmas in the corona are very hot, and the scale-height of the corona Hc ~ R. Powerful jets with Qjet ~ Lbol (bolometric luminosity) can form by the large-scale magnetic fields created by dynamo processes in the disk coronae of some radio quasars.
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