ner_tags: O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O B-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O
tokens: of alignment of the outer disc with the equatorial plane of the BH. Caproni et al. (2004) and Liu Melia (2002) show that the misalignment between the rotation axis of the accretion disk and of the Kerr BH can explain jet precession in AGN as well. The authors successfully apply this scenario to quasars, Seyfert galaxies, and Sgr A*. However, Nixon King (2013) show that it is unlikely for any type of BH system to explain jet precession due to the result from a combination of the Lense-Thirring effect and accretion-disc viscosity. In general, the angular momentum of the disc is not sufficiently large, in comparison with that of a spinning hole, to cause jet precession on short timescales. AGN accretion events are assumed to be uncorrelated and this would cause changes in the jet directions on comparably large timescales of yr. Chaotic accretion is discussed in more detail by King Nixon (2015) – the effect of self-gravity limits the mass of any accretion-disc feeding event. They suggest that additional disc physics is needed to explain any jet precession on timescales short compared with the accretion time. Lubow et al. (2002) investigate the shape of a warped disc around a Kerr BH. In particular, they investigate this system under conditions such that the warp propagates in a wave-like manner. One of their results is, that in low-viscosity discs around a Kerr BH, the inner parts of the disc are not necessarily aligned with the BH. Because the shape of the inner disc depends sensitively on the radial dependence of the disc properties, a changing accretion rate can cause a change in the inner disc warp even without effecting the tilt of the outer disc. This finding contrasts with that of, for example, Nelson Papaloizou (2000) . Lai (2003) discusses the warping of an accretion disc due to magnetically driven outflows as a possible origin for jet precession. In particular, the disk might be subject to warping instabilities and retrograde precession driven by magnetic torques associated with the outflow. The growth timescale for the disk warp and the precession period would be of order the radial infall time of the disk. Caproni et al. (2006) study four mechanisms (tidal, irradiative, magnetic, and Bardeen-Petterson) that can cause warping and precession in galactic and extragalactic accretion disks. Eight X-ray binaries and four AGN are examined. The paper demonstrates how difficult it is to constrain the precession mechanism. The Bardeen-Petterson effect can produce the precession timescales compatible with the observed ones in all sources of their sample. However, the work by Caproni et al. (2006) shows that due to the many observational uncertainties other precession mechanisms can not easily be ruled out. In addition, more than one mechanism might be at work in a given source. Another alternative explanation of the observed effects could result from a magnetically-arrested disc (e.g., McKinney et al. 2012 ; Tchekhovskoy et al. 2014 ). Using