Magnetic fields of active galactic nuclei and quasars with regions of polarized broad Hα lines

Abstract

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.

Can environmental effects spoil precision gravitational-wave astrophysics?

“…The largest magnetic fields can be found in magnetars and reach B ∼ 10 15 Gauss [56]. The magnetic field of the supermassive BHs that power active galactic nuclei is believed to be 10 3 − 10 5 Gauss [57][58][59]. We will take B ∼ 10 8 Gauss as reference value for a very large magnetic field.…”

Gravitational lensing by a rotating massive object in a plasma

“…The distribution of magnetic field around black hole and polarization in accretion discs of AGN is studied in Gnedin et al (2012). The dependence of the degree of polarization on the spin of the central black hole is studied in Silant'ev et al (2013b). The approximate analytical expressions for the polarization of the radiation from a magnetized disk around black hole can be found in Silant'ev (2002Silant'ev ( , 2005, Silant'ev et al (2009).…”

“…It has to be mentioned that estimates of magnetic fields for active galactic nuclei can be based on the observed polarization degrees (Silant'ev et al 2013a). The distribution of magnetic field around black hole and polarization in accretion discs of AGN is studied in Gnedin et al (2012).…”

Spectral distribution of the polarized radiation from standard accretion disks in Active Galactic Nuclei: Observational analysis

“…The magnetic field at the event horizon B H can be determined by equating the pressures due to the magnetic field and the accreting gas [25]:…”