Bardeen-Petterson Effect and Broad HI Profiles of Seyferts

Extract

A thin accretion disk could be not only the energy source of AGN but also the matter producing broad emission lines of Seyfert 1 type nuclei (Dumont & Collin-Souffrin, 1990 B). A possible mechanism for this is reprocessing of central hard X-ray radiation by the outer (at 10^2–5 RG, RG is the Schwarzschild radius), low-temperature regions of the disk. This mechanism is effective enough especially if the disk is a non-planar structure (a warped or twisted disk), when the outer parts could be directly seen from the centre. An accretion disk around a Kerr black hole could be twisted if the angular momentum of the accreting gas is initially not aligned with the rotation axis of the hole. Due to the differential Lense-Thirring precession of orbits around a Kerr black hole, a viscous disk is a steady but non-planar structure. This is the well-known Bardeen-Petterson effect (Bardeen & Petterson, 1975). Near the hole, at distances R>RBP, where RBP is the Bardeen-Petterson radius, the flow is aligned with the equatorial plane of the black hole, while at larger distances it is tilted to its initial orbital plane (Fig. 1).