The Black Hole Mass and Magnetic Field Correlation in Active Galactic   Nuclei
  The observed optical luminosity in 5100 angstorm and black hole mass correlation is used to probe the magnetic field of black holes harbored in active galactic nuclei(AGNs). The model is based on the assumption that the disk is heated by energy injection due to the magnetic coupling(MC) process and the gravitational dissipation due to accretion. The MC process can transfer energy and angular momentum from a rotating Kerr black hole to its surrounding disk. The relation of optical luminosity in 5100 angstorm and black hole mass as functions of the spin and magnetic field of the black hole is modelled. The model predicts that optical luminosity in 5100 angstorm emitted from the disk is sensitive to the strength of the poloidal component of the magnetic field on the BH horizon. Based on the observations of optical luminosity in 5100 angstorm for 143 AGN sources, we obtain the correlation between mass and magnetic field of black hole. And we compared out result with the approximate result between mass and magnetic field of black hole derived from the condition in the standard accretion disc theory. 
W. M. Zhang, Y. Lu, S. N. Zhang