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Abstract
Observations of accreting black holes often provoke suggestions that their jets precess. The precession is usually supposed to result from a combination of the Lense-Thirring effect and accretion disc viscosity. We show that this is unlikely for any type of black hole system, as the disc generally has too little angular momentum compared with a spinning hole to cause any significant movement of the jet direction across the sky on short timescales. Uncorrelated accretion events, as in the chaotic accretion picture of active galactic nuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In this picture AGN jet directions are stable on shorter timescales, but uncorrelated with any structure of the host galaxy, as observed. We argue that observations of black-hole jets precessing on timescales short compared to the accretion time would be a strong indication that the accretion disc, and not the standard Blandford-Znajek mechanism, is responsible for driving the jet. This would be particularly convincing in a tidal disruption event. We suggest that additional disc physics is needed to explain any jet precession on timescales short compared with the accretion time. Possibilities include the radiation warping instability, or disc tearing.
Highlights
Jets appear in all accreting systems, from protostars (e.g., Davis et al 1994) to active galactic nuclei (AGNs; e.g., Nagar & Wilson 1999; Kinney et al 2000)
To tap the maximum accretion energy, a jet produced in this way must come from the innermost part of the disk near the stellar surface, and so naturally gives a terminal velocity of the order of the escape speed
We have argued that the physics of standard warped disks strongly suggests that the Lense–Thirring effect alone is not a promising mechanism for explaining jet precessions, except possibly in rather rare cases
Summary
Jets appear in all accreting systems, from protostars (e.g., Davis et al 1994) to active galactic nuclei (AGNs; e.g., Nagar & Wilson 1999; Kinney et al 2000). The Lense–Thirring effect of a spinning black hole makes tilted disk orbits precess around its angular momentum vector at a frequency ΩLT = a(R/Rg)−3ΩK(Rg) (where a is the Kerr spin parameter, Rg = GM/c2 is the black hole’s gravitational radius, and ΩK(Rg) is the Kepler frequency at disk radius Rg), which decreases strongly with radius (Thirring 1918; Lense & Thirring 1918) This differential precession is communicated through the disk by its viscosity, which acts to co- or counter-align the disk with the plane of the hole. The inner parts of the disk quickly settle in the equatorial plane of the black hole and the outer parts remain misaligned, with the two parts joined by a warped region This is the Bardeen–Petterson effect (Bardeen & Petterson 1975; but note that the equations of that paper do not conserve angular momentum; see Papaloizou & Pringle 1983). We conclude that in a tilted disk propagating warps in the diffusive regime (α > H /R), the Lense–Thirring effect alone cannot drive repeated jet precession, unless the disk is torn into many distinct planes (Nixon et al 2012a)
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