2005MNRAS.357.1155Y

A toy model for jet powers and radio loudness of active galactic nuclei (AGNs) is proposed based on the coexistence of the Blandford-Znajek and magnetic coupling processes in black hole (BH) accretion discs. It turns out that both the jet powers and radio loudness of AGNs are controlled by more than one physical parameter besides the BH spin. The observed dichotomy between radio-loud and radio-quiet AGNs is well interpreted by the two parameters, the BH spin and the power-law index of the variation of the magnetic field on the disc. Furthermore, we discuss the correlation of jet powers with radio loudness of AGNs in terms of the two parameters. It is found that the contours of radio loudness are approximately in accord with those of jet powers for several 3CR radio sources, implying roughly that the stronger jet powers correspond to stronger radio loudness. In addition we discuss the correlation of the jet powers and radio loudness of AGNs with the position of the inner edge of the accretion disc. These results imply that the `spin paradigm' for radio loudness of AGNs might be modified by a scenario containing more physical parameters.