The jet power extracted from a magnetized accretion disc

We consider the power of a relativistic jet accelerated by the magnetic field of an accretion disc. It is found that the power extracted from the disc is mainly determined by the field strength and configuration of the field far from the disc. Comparing with the power extracted from a rotating black hole, we find that the jet power extracted from a disc can dominate over that from the rotating black hole. But in some cases, the jet power extracted from a rapidly rotating hole can be more important than that from the disc even if the poloidal field threading the hole is not significantly larger than that threading the inner edge of the disc. The results imply that the radio-loudness of quasars may be governed by its accretion rate which might be regulated by the central black hole mass. It is proposed that the different disc field generation mechanisms might be tested against observations of radio-loud quasars if their black hole masses are available.