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We present estimates of magnetic field in a number of AGNs from the Spectropolarimetric atlas of Smith, Young & Robinson (2002) from the observed degrees of linear polarization and the positional angles of spectral lines (Hα) (broad line regions of AGNs) and nearby continuum. The observed degree of polarization is lower than the Milne value in a nonmagnetized atmosphere. We hypothesize that the polarized radiation escapes from optically thick magnetized accretion discs and is weakened by the Faraday rotation effect. The Faraday rotation depolarization effect is able to explain both the value of the polarization and the position angle. We estimate the required magnetic field in the broad line region by using simple asymptotic analytical formulas for Milne's problem in magnetized atmosphere, which take into account the last scattering of radiation before escaping from the accretion disc. The polarization of a broad spectral line escaping from disc is described by the same mechanism. The characteristic features of polarization of a broad line is the minimum of the degree of polarization in the center of the line and continuous rotation of the position angle from one wing to another. These effects can be explained by existence of clouds in the left (keplerian velocity is directed to an observer) and the right (keplerian velocity is directed from an observer) parts of the orbit in a rotating keplerian magnetized accretion disc. The base of explanation is existence of azimuthal magnetic field in the orbit. The existence of normal component of magnetic field (usually weak) makes the picture of polarization asymmetric. The existence of clouds in left and right parts of the orbit with different emissions also give the contribution in asymmetry effect. Assuming a power-law dependence of the magnetic field inside the disc, we obtain the estimate of the magnetic field strength at first stable orbit near the central supermassive black hole (SMBH) for a number of AGNs from the mentioned Spectropolarimetric atlas.

We estimated the magnetic field strength at the event horizon for a sample of supermassive black holes (SMBHs) in active galactic nuclei (AGNs). Our estimates were made using the values of the inclination angles of the accretion disc to the line of sight, which we obtained previously from spectropolarimetric observations in the visible spectrum. We also used published values of full width at half-maximum of spectral line Hβ from broad-line region, masses of SMBHs, and luminosity of AGNs at 5100 $\mathring{\rm A}$. In addition, we used the literature data on the spins of SMBHs obtained from their X-ray spectra. Our estimates showed that the magnetic field strength at the event horizon of the majority of SMBHs in AGNs range from several to tens of kG and have mean values of about 104 G. At the same time, for individual objects, the fields are significantly larger – of the order of hundreds kG or even 1 MG.

The spin of central black holes with intermediate masses in globular clusters is determined using the well known relationship between the kinetic power of a relativistic jet and the observed radio luminosity of the region closest to a central black hole. The estimate of the magnitude of the spin is based on the knownBlandford-Znajek mechanism. The magnetic field near the event horizon of a black hole is determined using a magnetic coupling mechanism that assumes equality between the densities of the magnetic and kinetic energies of the accreting gas (the Magnetic Coupling Model). The rate of accretion M is derived on the basis of the Bondi-Hoyle mechanism.

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