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Determining the spin of Kerr black holes in AGNs

Bardeen-Petterson effect has been shown to dominate the dynamics of a large sample of precessing (tilt) accretion disks and jets in galactic and extra-galactic astrophysical sources. It is a general relativistic effect in which the outward angular momentum carried away by the accretion disk viscosity is counterbalanced by the inward frame-dragging due to Lense-Thirring effect. This stretching struggle produces a stress that fragments the disk at the Bardeen-Petterson (BP) radius. While the disk outter part keeps its original orientation w.r.t. the central black hole (BH) angular momentum axis, the disk inner part is driven to get aligned with the BH spin axis on a time scale dictated by its structure and the spinning rate and mass of the Kerr BH. Such a self-gravitating inner disk becomes a steady (continuous) source of gravitational radiation. Here we show that one can measure the spin of the Kerr black hole from the shape of the warped accretion disk, the precession of parsec scale jets, and/or from the structure of the radio-galaxy at kiloparsec scales. We exemplify this our procedure with the galaxy NGC 1068. Our analysis suggests that NGC 1068 has a retrograde spinning BH.

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